Maybe. Anime logic would suggest so (killing an enemy will inherit their power). I'm assuming Witch Factor would be the power to use moves such as Invisible Hands. Whether the whole idea of hands is tied to Sloth's Witch Factor, or whether the factor simply manifests as an individualised power (might not be hands for Subaru) is yet to be seen.
I kind of hope he doesn't get it though. I like the idea that someone else obtained it because Subaru didn't know about the power beforehand and left it lying around. You know, consequences and all.