Neville Longbottom was born under the cloud of Voldemort and a prophecy foretelling his downfall. As it turned out—for reasons known only to Voldemort—that prophecy and the Chosen One ended up being Harry Potter, not Neville Longbottom. This will lead to Voldemort’s downfall because we can assume that Alice probably would not have had the chance—like Lily—to step between her son and Voldemort. Although it worked out this way, Neville’s destiny is just as determined by that prophecy as Harry’s was.
After the fall of Voldemort, the Order and Wizarding World had to deal with Voldemort’s Death Eaters who were as much of a threat as Voldemort himself. When Neville is over a year old, his parents are tortured into insanity by Bellatrix Lestrange and pals with the Crucio curse, in order to—according to Bella at her trial—discover the location of Voldemort.
There are a few questions that I have at this point. First of all, why did Bella and pals think that the Longbottoms knew about Voldemort’s location? Why would the Longbottoms know that and not the Death Eaters?
In other words, I don’t think that that is why the Death Eaters attacked the Longbottoms, or not entirely. When they say that they wanted to know the location of Voldemort, I think that they really mean (but can’t say it) that they want to know the location of part of Voldemort. By that, of course, I mean a Horcrux.
Bella suggests at the beginning of HBP that the Dark Lord trusted her with something very important (34), but that Lucius did something with it. She could be talking about the prophecy. But she could also be talking about a Horcrux, like (or maybe even) the diary of Tom Riddle. Though we doubt that the Death Eaters knew about the Horcruxes, they were obviously entrusted with them. RAB had the locket; Lucius the diary. What if the Longbottoms had stolen or obtained a Horcrux from Bella and she wanted it back?
Here’s another question that I have: How were Bella and pals caught? Mr. and Mrs. Longbottom obviously survived but were in no condition to either defend themselves nor testify against them. In being alive, they were evidence. Evidence that should have been disposed of. It would be safe to assume that Bella and friends were caught in the act and stopped. The question is, by who?
Why, Snape, of course. But I’ll come back to that.
Was Neville there during this horrible experience? I believe yes. Neville acts like someone who is under a memory charm, someone who’s brains have been magically addled. While his parents were talented wizards, Neville is “nearly a squib”, sort of, but not really. It seems as though Neville is a powerful wizard, but he is handicapped by two things: his memory and his own fear (i.e. self conscious).
Neville acts very much like two other people who have had their memories altered: Bertha Jorkins (who, under a memory charm loses other memories as well, who is bumbling through her job at the Ministry, and yet was hired by them) and the thoroughly confused sneak Marietta. In the first book, Neville gets his infamous Rembebrall (which very tellingly turns red the first time he touches it—implying, like the sneakascope, that there is something continually wrong in the situation in which it was presented). Neville says, “The problem is that I can never remember what I’ve forgotten.” In a lovely scene, which I think is thoroughly foreshadowing, Harry retrieves Neville’s Remebrall from a cruel Draco. Just in case we may have forgotten that little seemingly unimportant toy, OOP brings it up to tell us that Neville has lost it.
In fact, Neville’s memory gets even worse when he’s emotionally stressed, particularly in the presence of certain people. One, obviously being Bellatrix Lestrange. He makes a couple bad mistakes during the Battle for the Ministry (i.e. bad aim). But the oddest incident is after Sirius Black falls through the veil and Neville asks Harry if “that man, Sirius Black was [Harry’s] friend”. Neville should know that Sirius Black is Harry’s friend. Sirius Black is the whole reason that they all went to the Ministry! Harry is screaming about it, right in front of Neville, only an hour previous. That's a bad memory. In my opinion, a memory charm has been reinforced on Neville while in Bella’s presence. It was done either by someone, or within the spell itself. I doubt that it was Bella (not only does she have no reason, but she doesn’t strike me as that smart). In other words, Neville’s memory problems are oddly patchy. They come and go, strengthen and weaken. But they are closely related to his emotions.
Paradoxally, Neville’s emotions also can help him to become more focused. Emotions in the Potterverse obviously can focus and strengthen a person (i.e. Harry with the justified Unforgivable Curse on Bella) and can even help someone through off mind-control (Harry with Voldemort). Neville becomes incredibly focused in DA, especially when he is highly aggitated by thoughts of Bella and friends. That's not the only person who continually aggitates Neville.
Because Snape is the other person that seems to affect Neville, both magically and emotionally. Neville has a terrible memory in Potions. He’s constantly being bullied by Snape. Why does Snape bully Neville so much? The only person he bullies more is Harry. Not Ron, nor Hermione, get it as much as Neville. Snape is a big fat meanie, but he does work with a purpose. We know why Snape bullies Harry. I’m not talking about all that lame schoolboy grudge crap, either. I’m talking about riling Harry up emotionally to prepare him for the big show down with Voldemort. If Harry didn’t have Snape provoking him, if Harry didn't learn how to deal with Snape and his emotions, Harry would break down much worse when he faces Voldemort. Not to mention, Snape is trying to teach him to keep his mind and mouth closed. So, yes, I do think that Snape has a reason behind bullying Neville, too. Snape’s all about the tough love. Unfortunately, Snape has not created the desired effect in Neville Longbottom. That is because Snape and Neville have a past.
If I had to venture a guess, I’d say that Snape broke in on Bella and pals torturing the Longbottoms. They could even have been using Neville as a way to get information from his parents. Snape, still under cover, puts a memory charm on Neville, gets them all to leave, and passes the truth onto Dumbledore. But now, Snape (along with Alice and Frank Longbottom and the Remembrall) needs Neville to remember where that Horcrux is. Snape is trying to get Neville worked up so that he will remember, trying to get his emotions to do what they do when he learns about the mass breakout from Azkaban: focus him. But it doesn't focus Neville because he is addled by a spell. Neville is petrified of Snape, and this could quite easily be residual emotions, remembering that Snape was there that night. It's subconscious, though, and Neville doesn't know where these feelings come from. It’s not until Bella and her friends break out that the conscious side of Neville kicks in and becomes the focused killing machine. It's not hard to believe that Snape would do this, nor to forgive him. It’s much less cruel than whatever Voldemort did to Bertha to get her information, which left her so damaged that she was killed. To again compare Snape’s treatment of Harry with that of Neville, look at Snape’s last actions at Hogwarts. When he runs by Neville, something happens to the kid. Neville is talking kind of funny, lying on the ground sick. Not injured, but sick.
There are two more relics that we have to take into consideration when theorizing about Neville. One is of course Trevor the toad, Neville’s uncool, frequently trying to escape, familiar. Also, there was that time that Snape tried to poison Trevor and was thoroughly disappointed when it didn’t work out. There’s always a reason for Snape’s actions, even when they are cruel. Why is Trevor always trying to escape? Why is Neville so obsessed with holding onto him? Also, it certainly wouldn’t be the first time that a pet ended up having vast significance in the Potter series. Scabbers was Wormtail, Crookshanks proof of Sirius’ innocence, etc.
If I had to venture a guess, I’d guess that Trevor is a key to the Horcrux. He might be someone or something transfigured that belongs to Voldemort, explaining why he’s always trying to escape. Neville, therefore, would be Trevor's keeper. Trevor could also be the sourse of Neville's memory charm. This would explain why Neville holds onto him so dearly. He doesn't want to remember.
And the bubblegum wrapper? Sometimes a bubblegum wrapper is just a thoroughly tragic bubblegum wrapper. But I can’t help but imagine that Alice is trying to tell her son something, desperately trying to remind him of something. Part of it, I'm sure, is that Neville needs to do something. He needs to get his revenge. But wouldn't it be a much more satisfying ass-kicking if Neville didn't kill Bella, but actually killed 1/7th of Voldemort's soul? Eat that!