You only have 3 kinds of multiplayer
Multiplayer matches: Team vs team or FFA (Think halo or BF3)
Co Op: You and a few friends go do the campaign or similar sections (Think Borderlands)
MMO: You, your friends, and a dozen other people, or maybe even hundreds go out to do objectives, and also have mutiplayer matches. (Think WoW and GTA:O)
Are you telling me you would rather have GTA:O just be nothing but deathmatches?
It's already 100% death matches every time you login to multiplayer--regardless of what session you login in to.