Angry with your RE5 co-op partner for not healing you when you were on the brink of death last game? Show them what for in the game's Versus Mode DLC, but be prepared to pony up some digi-dollars for it.
Friendly fire is optional in Resident Evil 5's co-op campaign, which should come as a relief to anyone tired of "hilarious" griefers in co-op games like Left 4 Dead. But come on, let's all fess up: it's fun to shoot your teammates in the face sometimes. For that, there'll be RE5's Versus mode, a four-player competitive multiplayer mode that'll be released as DLC post-launch and run you $4.99 on PSN and 400 MS Points on Xbox Live.
There are two modes in Versus. In Slayer's Rule, you compete with other players to see who can kill more enemies. The highest score at the end wins. The other mode, Survivor's Rule, is a team deathmatch mode where the goal, obviously, is to kill the other people. In either mode you can play as RE5 heroes Chris, Sheva and other "secret characters," which assuredly means that Wesker will be selectable.
Anyway, if you can get over the fact that we've gotten to the point where we have to pay for a competitive multiplayer mode, look for Versus some time "post-launch." That means expect it soon but not too soon, you never know. Capcom likes to be a tease about these things: they said the Street Fighter 4 Championship Edition patch would be "post-launch" too, and that's still not out.