the last book i read

The last book I read was a good book. I can not remember the title of teh book.
it was about a guy that is poor and becomes an assassin. He does work for this guys and later ends up becoming his enemie because there were a set of rules he did not follow. They become enemies and teh assasin kills his leader.
The boojk was part of a trilogy. I read it in jeuvenille hall. it was about 400 pages long and i cant remember how it ended. i think it ended with him killing the leader but i can not remember.