Last week I set up a webcam that will trigger on motion
This is actually a much bigger issue than the theft of your food. Depending on where you live it could very well be illegal to film someone without their permission, and the management at your company is almost undoubtedly not going to like it if they find out you were filming in the office without their permission. Most companies expressly forbid this, because of the risk that something confidential may be caught on film and distributed outside the company, never mind the legal risks and the damage to your relationship with your co-worker should he find out you were spying on him. I also suggest changing your display name on this site, since if it is a real name it could be used by someone at your company to ID you and get you in trouble.
Now I know that it's only an occasional package of peanut butter
crackers, something I would happily give him if he asked for it, and
it certainly doesn't amount to much, but it makes me wonder if there
is other stuff that he has picked up that I don't know about, or if
he's done it to others in the company.
In general, innocent until proven guilty is the best approach. You shouldn't just assume he was taking any more things if there is no evidence of this.
My questions here for consideration are, am I obligated to report this
to my boss or to HR?
Absolutely not. Not only will "theft of crackers" seem like a trivial thing to report someone for, and the sort of thing that can generate whispers about you at the office, but you would have to admit that you filmed the whole thing, which will cause a lot more trouble for you than your co-worker.
Would it be better to confront him about it -- and thereby creating a
very uncomfortable atmosphere in which to work?
You should talk to him about it, but not in a way that implies that you KNOW he did it (otherwise he will wonder why, and the camera issue surfaces again). Simply inform him that you noticed that your supply of crackers was dwindling, and ask him if he knows whether anyone has been taking them. This way, you're not confronting him directly, but he'll get the message that it's not appreciated. If he admits to it, let him know you'll gladly share some with him, but he should ask you first if he wants some. If he stays silent, don't pursue the subject. Chances are he knows he did something wrong, and will stop. You can take further action by not keeping your crackers at work, and choosing to bring a package there from home every day just for you. This should prevent further thefts.
Should I forget about it? If I do not report it, would there be some
liability for me? If I do report it and they take corrective action,
do I open myself up to criticism or possible retaliation from other
employees for being a snitch? What if he gets fired? I don't want him
to lose his job. There are so many things to consider.
There shouldn't be any liability for you for not reporting it, seeing as the crackers are your property and it's up to you whether you want to report it. If it was company property on the other hand, it may be a different matter. It certainly won't help your relationship with your co-workers if you report someone to your manager for stealing your crackers. It's a pretty minor offense. He wouldn't get fired if he did get reported unless the managers are extremely unreasonable, but you could if you had to admit to filming the incident, so it's best to keep quiet about it all.