Ben vs. John?

[RoccoRamsey]

Ok first post here. Big time lostie, whatched the show from day one. Ok so im confused, why does Ben kill Locke and acts like he will never see him again, but in previous episodes we see Ben and Jack dragging around Locke's casket and Ben knows that Locke has to be on that plane for this to work. My theory is that Ben knew that maybe John would not eventually want to go back so he had to kill him to get him on that plane. Need some input!