Board Thread:Legitimate Canon Questions/@comment-13895380-20161123045622/@comment-25176701-20161127001620

Liam can't blame Corey for following Theo because it was Theo who brought Corey back to life (even if it was for his own selfish reasons) when Liam and the rest of the pack failed to protect him. Corey felt like he owed it to Theo, and was intimidated and manipulated into staying part of the pack.

Corey also protected Mason from the others (because he didn't know what they would do to him) by turning him invisible and hiding him out in the sewars when he found out Mason was La Bête du Gevedaun.

Besides, Mason is right. Corey couldn't exactly help fight Sebastian. All he could do was turn invisible, and even invisible Sebastian would still be able to sense his presence. If Corey had tried to help, he would just make it harder for everyone else because they would have to focus on fighting Sebastian AND keeping him safe, since he couldn't protect himself well enough.