Ray had complete control over the entire "hive" base. So, the moment he realized Stephen's ability relied on observing through mirrors, he immediately made his exit.
He had already taken out Corey with a spatial severing cut and stashed the corpse in his otherworldly storage space.
Of course, if you're going to kill someone, you don't just leave their abilities lying around for free—why let the enemy benefit?
At this point, Susan's faction could at least be considered "half an enemy," he figu