I've already explained why.
He's calculating, I'll give him that, but his behaviour a few times has been very bad. He's turned on Claire a few times this series even though they've got on well in the episode itself. This was especially evident in the episode when Simon got fired. He realized that going for Simon was a mistake as Al liked him and simply turned on Claire. Alan DID fire Simon though, and Alex exposed himself as a rat. As I say, he'll be dealt with, it couldn't be tonight, as there was far more serious culpability on the other team members.