Read this about 7-10 years ago
Futuristic. Assassin kills people, then has his memory wiped (voluntarily). He moves a few beer bottles in his fridge to keep count of how many people he's killed
Scientists discover a worm hole, and through that, a parallel Earth
Assassin is asked to steal a spaceship, investigate the planet, and kill a scientist there
Some complications occur, and he meets a woman
Half way through the job, his time expires, and his memory is wiped out, and he forgets the mission and the woman he met. He then tries to kill her, till she tells him of the secret of his beer bottles. He can't remember the mission, but decides to trust her.
They finally do kill the scientist, I think.
At the end of the book, the assassin finds out
that there are many parallel Earths, and he is from one of them. The main Earth has been killing scientists on the other worlds to keep them backward. The assassin decides to go back to him home planet and destroy the corrupt system