In Dead Rising 2: Off The Record for the PC, PS3, and Xbox 360, the psychopath costume will not only make you look scarier, it will also you to do more damage with hacksaw movie type weapons (machetes, chainsaws, knives, cleavers, etc.). Secondary attacks will also have different animations, and you will take less overall damage.
To get the Psychopath Clothes, you need to defeat Carl in the Mail Order Zombrex mission.
Check out my other guide if you’re looking for the whole Psychopath Costume set.