Why is gears of war bad for kids

Gears of War is rated M for Mature.
It contains a lot of blood, violence, shooting, chainsaw to the face kinda action.
There is some coarse language and the apocalyptic outlook of the game could be pretty depressing for a kid.

That said, it is a fantasy game, and if you make sure your kids are aware of the difference between these scenarios and reality, the chance of them taking a chainsaw to someone is pretty slim.