Yes, he watched them suffer is actually what he did.
How can you watch to "make sure they didn't suffer" while watching them suffering? What would he have been watching for? How would he know if, while watching, they suddenly began to suffer? And what did he mean he would have done if he saw that they were suffering while he was watching to "make sure they didn't suffer'? What could he have done then? Stab them again? To end their suffering?
That was the most ridiculous thing he said, imo, that he watched until they died "to make sure they didn't suffer". There was nothing he could have done for their suffering except to quickly end it by stabbing (or shooting) them dead. They were already dying. They were already suffering. He would have been able to plainly see that they were suffering, if they were still alive after he used his box cutter on their necks. He wasn't about to get them any help. So what did he mean by that? What would he have done to help end their suffering, except to make death come more quickly???
He stayed there awhile after he fatally wounded those girls for no reason other than to watch them die. And then he left.
Sickening.