I think CW was defective so nothing he does or says is explainable or excusable, imo, so I'm just speaking of the potential psychology of why he put the girls he was so "devoted" to into the oil...
He seemed to have good feelings about the oil business, or "tar heels" ...if that's the case maybe to HIM putting them there was not only a good place to hide them but also it's not so bad of a place for them to be, in his twisted mind. Some are saying he must have hated the girls and that's why he put them there, and SW in a grave, albeit a shallow one. I think the opposite is true. But maybe if he had time he would have buried them all, or if there was room in the tanks SW would've been in there too...who knows, but the thing is, he's guilty as sin of killing all of them in cold blood. If he had snapped he never snapped out of it because this was premidated, imo. He just ran out of time. (And thank you, NU!)