I'd forgive him because it's only something in the past. People change and people should realize that. They should also realize that hating something will just get them stuck in the circle of things. If someone believes they should die for their actions, that's negativity and negativity just causes more stress and if someone is that someone who causes more stress just think about their actions. Watch the actions of one who hates, harasses, and assaults 'em. Are the people any better than his action's in the past? For me, people aren't strong if they beat someone down just because of their past doings. People are stronger when they forgive and try to change the person that's performing the wrong doing. Also, smarter than a person who beats them down because heck, if you can change a guy into being the nicest person in the world, then you can change anything.
Anyways, this guy changed into feeling guilt. Guilt is a good emotion to feel after you've done something wrong, but if you feel it for too long, you might feel like destroying yourself. It's not good in that way. Like you said, it's been two years of him suffering. No one deserves to suffer for that long. I mean it.
Other people might argue that, "Oh, he should get tortured because he tortured something else in his past! An eye for an eye, a tooth for a tooth!" but I disagree because one might end up getting revenge and fighting back. What if that guy that hurt the animal gets up and starts to hurt you back after you hurt him so many more times than he hurt the animal? If you realize the circle of revenge and get out of it, the more happier you'll be in life. Also, the more smarter you'll be.
In finality: Forgive and change. You're stronger if you're the type to.
I hope this makes sense.