But then CSM tried to have Rat Boy killed but didn't succeed. Now Krycek turned on him and works for himself.

Krycek may have killed Mulders father and Scullys sister.

For awhile Krycek was possed by the black that who left him trapped in an abandoned missile silo in South Dakota.

Bad luck follows this man doesn't it! In the 4th season episode Tunguska, while he and Mulder were in Russia his left arm was cut off so he couldn't be infected by a virus (the black oil?) Since then he has had a prostetic arm but sometimes Krycek forgets that he has no left arm so he can move it very well.

Krycek wasn't in season 7 much at all. But in the season finale, Requiem, we find him in an African jail and is bailed out by Marita. By the end of the episode, he is back up and going. He decides to kill the Cigarette Smoking Man who is dying already from his smoking! He pushes him down the stairs in his wheelchair. Is he really dead? Only time will tell.