Few guesses....
1) Emile Blonksy. I know they found abomination dead, but remember that Doc sampson is involved...he could easily switch minds into a different body. Which would explain why red hulk's first act was to kill "Abomination".
2)Betty Ross. We assume red hulk is a man, but who knows. Perhaps after abomination "killed" her she was brought back with gamma radiation... Also, it would explain why Loeb went after Rulk with a group of super women. Trying to throw in some irony when people found out who Rulk really was. The drive to kill hulk could stem from a belief that destruction of the beast would free the man inside.
3)Banner. Either another split off Hulk persona story, OR literally just Banner. Lets not forget that Rick Jones is now Abom....and (unless something has happened since I stopped reading the book) we only ever saw Banner and then the Green hulk. Never saw Banner become the green hulk. Perhaps the green hulk is the split off persona, OR someone else entirely that Banner feels responsible for and needs to stop. As far as I know Rulk has never killed anyone. Rulk has claimed not to be Banner though, and the betty bots didnt work on him so I think this is a long shot.