Hakaiman is a top-down action stealth game. You control a commando who on each single-screen map have to complete various objectives, usualyl while killing a large number of enemies all on your own.

You control your character with the arrow keys. Shift is the general action button, which will fire your gun if you have it armed, and punch otherwise, and perform actions if you are in front of something upon which an action can be performed, such as a key (grab) or a team mate (follow/stop). Z is reload if you have your gun out, and grenade throwing otherwise. X changes your weapon.

The whole game is in Japanese, and there's no English translation, it should be reasonably clear what you need to do, otherwise just running around trying out things usually works.