"You's a killer Bunny," says Junior during one scene.
And that's putting it lightly. The guy is pretty much a psycho, and for a lot of reasons. But that's to anticipate matters. We don't know for sure where Bunny is from, but he definitely seems to be from somewhere down south. He likes his Budweiser, and his Indy Racing, and he really doesn't care for marijuana (he's convinced the Vietnamese are lacing it with stuff to make the U.S soldiers pacifists).
Now, as for Bunny being a psycho, when the platoon enters a village and uncovers a cache of weapons, Bunny decides to take matters into his own hands in one of the huts. After Taylor decides not to kill a boy, Bunny steps in, intimidates him, and then beats him to death by hitting him in the face with the butt of his rifle. Afterwards, he acts like somebody who has just had the thrill of their life: "Holy s*** you see that f***ing head come apart man? I've never seen brains like that before man."
What else makes Bunny psycho?
Well, for one thing, the fact that he likes Vietnam. Yikes. His exact words to Junior before the final battle (in which he is killed) are: "I told the padre the truth man I like it here. You get to do what you want, nobody f***s with you. The only worry you got is dying, and if that happens you won't know about it anyway so what the f*** man?" Bunny really doesn't get it. Sure, he gets it that death is a real concern (but he's not worried about it), but for him war is just a big playground—one where "you get to do what you want," like kill innocent people, rape innocent girls, and the like. It's kind of a good thing that he doesn't make it, so to speak. After all, he doesn't feel like, as he says to Junior in the same speech, they've done anything wrong.