He's not stupid and he's already seen how this is going to end. He's been around for longer than you would think and he's heard more than a few whisperings about the Volturi in the darkness of back alleys between hunts and in the depths of forests after the burials of hikers foolish enough to wander off course.

The Volturi will come and no matter how many witnesses the Cullens manage to gather, Aro will find a reason to destroy them and all those who sided with them. They will find their end sooner rather than later and Alistair's not the kind to go sacrificing himself for a bigger cause.

Instead he runs and leaves them behind.

They're all dead men already.