In the book mice and men who dies?

Lennie does. George shoots him while Lennie is looking at the trail head by the pond.

Curley's wife does. Lennie breaks her neck.

Candy's dog dies. The ranch workers bribe Candy into killing the dog as it is useless. Carlson takes the dog outside and shoots it.

Lennie's pup dies. Lennie pets the pup too hard and breaks its neck.

Also, lot's of mice are killed in the book. By Lennie, again petting them too hard.