I haven't seen this theory posted before, but MAYBE (although seemingly unlikely) sweet revenge is when one of the players becomes infected by the Astronaut Zombies (if that's what they can do as well), they have to attack and try to infect the other players in the game since they didn't protect you in the first place.

However, it could also be a 2nd Easter Egg after you complete the first one and can only be done after the first one.

What do you guys think?