Remember when he punched him on the elevator and told him this is for Maris? Why did he consider his responsible?
shareHe doesn't hold Tom responsible for her death - Tom called Maris a "bitch" a few minutes prior to the elevator.
shareBecause Tom didn't pay the ransom, it screwed up his plan so he had to kill everyone else to get the reward instead.
Promise me, no matter how hopeless things get, keep on trying, OK? Keep coming chin-up, OK?
But he killed Maris in self defense. Maris was going to kill him. In her case it was self defense.
shareMaris only tried to kill Shaker because she wanted to turn herself in and because Shaker had turned on the other two guys. None of that would have happened if Mullen hadn't put the bounty on their heads. Hence him blaming Mullen for Maris' death.
shareMaris tried to kill Shaker either because:
A) She had a change in heart and wanted to come clean
or
B) She was going to turn on the 3 of them and collect the reward for herself
I'm leaning towards A because she could have killed Sean and she didn't and there is the "you ruined my life" line.
Because sociopaths always blame someone else for their own bad behaviour and for any misfortune that is a consequence of their behaviour.