it had to be the void...the way murph was running from that thing gives you that impression, where as with the prisoners, even tho he might've been scared or frieghtened (spel) he still managed to fight them...and like you mentioned, the camera moves in such a way that would suggest it's something more than your average enemy..
Couldn't be.The Void can go straight trough walls, like said here before. Besides, it would just suck the door, not punch it. And The Void gives off those weird particles and distorts the light near it.
and it wasn't the boogeyman either, because Murphy killed him in the monastery, and the noises that he makes are a lot different. Besides, the Boogeyman is a lot bigger than the prisoner juggernaut, he wound't fit the vent, specially with his hammer.
So it was probably a juggernaut or a different monster.