There's a new mode in Resident Evil 6, and it lets you enter other people's games — and kill them. Dubbed "Agent Hunt", it's available after you've cleared the game and allows you to play as a RE6 creature — presumably a monster or zombie — and take on the game's protagonists.
Agent Hunt is only available on specific stages. And Agent Hunt players can use voice chat to coordinate with fellow Agent Mode players who have also inhabited the game. The player whose game has been infected with the mode will not be able to hear the voice chat.
When Agent Hunt creatures enter your game, you'll know; however, you won't know which creatures are Agent Mode players and which ones are computer controlled. Though, if you are getting unnecessarily trolled, then, yeah, you're probably dealing with real life jerks.
For those who do know what their play interrupted by Agent Hunt, it's possible to switch it off.