What is the meaning of 「さいってー」 in the following dialogue?
Context: before this scene, the character on the left has just explained to his accomplice on the right why he decided to betray and kill some of their friends, the scene ends after the dialogue above. I know that さ means something like "well" when used alone, but I've never heard of さいってー. Is it a synonym of さて?. Thank you for your help!