Clearly those claiming animal abuse are upset, and rightfully so, but try to get past that for a second and really look at the story rather than that one element.

These boys, especially the blonde, get sick amusement and a sense of power/dominion from punching these animals, and don't care if they (the animals) are hurting. The blonde one has deluded himself so much by thinking that these animals are going to heaven, that he doesn't care about their pain. He even deludes himself to the point where he understands that they're in pain, but he doesn't care. He enjoys it, and he is trapped in a memory. He is trapped in the memory, wherein his mother died in a burning car, because he couldn't punch open the window to save her.

He makes it his mission to become powerful enough to where nobody can stop him, to where he can save his mother in his nightmares and change the nightmares into dreams.

But- when they start fighting bigger game, such as wolves and bears, and start taking hallucinogenic mushrooms, the blonde one (Shall we call him Austin, from now on?) is so far gone that it's not about saving anyone, it's about power, and dominion over everyone and everything.

The brunette (Let's call him Steve) originally goes along out of a sense of mercy, but then gets sucked into it out of guilt, and because Austin is blackmailing and guilt tripping him. By the end, Austin turns on Steve and is Steve, after Austin kills a man (The Park Ranger), thereby becoming a true murderer, and when Steve calls Austin sick for this and threatens to call the police, Austin fights Steve. Steve, being as powerful as Austin and also being snapped out of the delusion (as Steve was not as far into delusion as Austin was) fights Austin in self-defense, and ultimately kills him.

The animals remove the corpse of Austin from the crime scene, as an act of vengeance, and a way of telling Steve that he is forgiven, but he is not to come back or he (Steve) will face the same fate as Austin.

It's not just about animal abuse, it's about the beginnings of a possible serial killer, a boy who went to far for personal redemption and went over the deep end of what is considered "normal" or "acceptable" behaviour and killed an innocent man, who was simply doing his job, but posed a possible threat to him (the boy) and his "activities".

Abusing and murdering animals is an early warning sign of a potential psychopath, sociopath, or serial killer, and that's what Austin was going to become.

It played a small, if also cruel, part in a beautifully told story.