Melissa's death in the UK edition. After having a bag pulled over her head, she gets her eyes ripped out, and is subsequently dragged kicked and screaming down the hallway by her bleeding sockets.
In general, the deaths in this movie can considered much more brutal than your usual slasher, as many of them involve the victims being strangled and suffocated with plastic bags before they are killed, giving an unnervingly sadistic quality to them.
Billy strangling his abusive mother and beating her with a rolling pin? Awesome. Making cookies out of her flesh? Creepy... or Black Comedy, depending on who you ask.
Everything about Billy abducting/attacking Agnes, who at that point is a little girl.
Specifically; Agnes hears a rustling noise behind the christmas tree, she peers through the branches, and Billy is staring right back at her, which a lot of first-time viewers miss. When their parents hear her screaming and come back down, he's just gouged one of her eyes out with a tree ornament, he then pops the eye into his mouth, before taking out both his parents.
