What are some good zombie or army/shooter type games for the PS2?

A really good shooter game would be Syphon Filter: The Omega Strain. It's a little challenging, but it's actually very cool and has lots of action and obstacles in it. Rated M, but it really isn't that bad. Just lots of shooting and quite a bit of blood and death. There's also Syphon Filter: The Dark Mirror, but I've never played that one.

For a zombie AND shooter game, I would definitely recommend Darkwatch. It's actually an amazing game- your character is a criminal/cowboy sort of dude who attempts to rob a train in the beginning of the game, but a ferocious vampire ends up being on the train and bites him. He himself becomes a vampire, but a good one, thus being forced to join the Darkwatch; elite fighters who strive to rid the world of the evil it is plunged in.

Awesome game play, cool enemies, challenging bosses, and lots more. Rated M, as well.

Hope you try them out :)

Black is a really good shooter, and it's cheap too. Only thing is that it doesn't have multiplayer and is a bit short.