I was thinking about this. after reading about Ben and Widmore's Rules. Ben saying Widmore changed the rules when he had Alex Kiled. and Ben saying he can't kill Widmore because of the Rules. alright. Rewind to when Ben was with the others, and Widmore was the leader. Widmore ordered Ben to go kill Danielle. Ben returned with the Baby (Alex). Widmore was mad because he told Ben to kill her, but Ben refused.
Widmore then said that It was Jacob's and the Island's will that the baby be killed. Ben then said that if it truly was the will of the island then Widmore could do it himself. And Widmore walked away.
SO. I believe that Ben saying Widmore changed the rules was because he had Alex killed, which was against Jacob and the island's will. Widmore took it into his own hands and did something against the "Rules". Ben realizing that Widmore was basically cheating.
Then to Ben talking to Widmore in the room, i think that Ben could've killed him, but didn't because it would be cheating and he knew he would be punished for it. So he decided to obey the rules and just kill Widmore's daughter instead as revenge.
Now Ben killed Widmore, because i don't think he beleives the Rules have much hold anymore. Jacob's dead, MIB is taking over and Widmore came back to the island. I think Ben just thought that everything has changed and he followed the Rules but got nothing in return, except a dead daughter, and snapped. Just like when he killed Jacob.
Ben may have been sneaky and 'bad' but he followed the Island's rules, only to realize it lead him nowhere...