That confrontation just before the scene with the bear. And of course, the bear itself.
In particular, right after Amos wounds the bear's shoulder. It roars in pain, then looks down at Amos with a Slasher Smile that the Joker would be proud of.
It gets even scarier on Amos' part: that was a protected forest, he was never supposed to be there in the first place. There wouldn't be any other people for miles around. He is essentially Alone with the Psycho in the middle of the wilderness, and this bear could snap his bones like they're toothpicks. Thank goodness Copper was there to come to the rescue!
The confrontation between Tod and Copper at the den. You want to see what a realDeath Glare looks like? That kind of snarl is what an animal does when it intends to defend something to the death - and no less.
The look on Copper's face is no better. Let's put it in perspective—he spent the better part of the movie looking like a happy, lovable non-anthropomorphic Goofy. Then shit happens, he blames Todd for it, and now he's looking like a straight-up Hell Hound as he tries to drag his former friend out of his own home and kill him.
That isn't even the worst part. Imagine a childhood friend that you grew up with? Forming a bond so close that the two of you believe nothing will break it. Then as you grow up you grow apart. Then later your childhood friend hates you and the next thing you know you and your former childhood friend are fighting to the death.