Here are the main explanations I've seen for Desmond running Locke over in the FST:
- Desmond is trying to give Locke a breakthrough between realities by giving him a near death experience.
- Desmond is trying to give Locke a breakthrough between realities by putting him into close contact with his constant, who is none other than Jack who now has to operate on him.
- Desmond thinks that Locke (not MIB) pushed him down the well in the OT, and is retaliating in the FST.
I'd like to propose a fourth option:
- FST Locke is MIB's ultimate escape off the OT island. Desmond is trying to close this gateway by killing FST Locke.
There's been recent discussion about whether FST Locke is MIB. My personal feeling is that we've been seeing some elements of Locke in OT MIB ("don't tell me what I can't do!") and hints of MIB in FST Locke ("then maybe you should be principal, Ben"). What if MIB's quest to get to the airplane on Hydra island is a red herring, and he actually has to accomplish something that will cause his being to shift completely from one reality to the other? What if "home" for MIB is not only someplace off the Island, but in the FST timeline where Jacob hasn't been mucking around with the past? If so, then FST Locke himself may be MIB's escape from the Island. MIB's goal is to completely possess Locke in the FST, at which point he can abandon the OT.
Why do you think Desmond ran Locke over in the FST?