A being who cannot die or has not seen death has no concept of it, therefore would not think its actions (should they be threatening) were wrong. No two people can coexist in one Nexus. Nexus is the individualized perfection of existence. Where people my have Nexus's that are the same, they could not coexist within one single Nexus. It is a contradiction in itself. Someone from the future is transported to the past where he comes in contact with someone from that time, and inadvertently reveals to him how and when he will die accidentally.
Now, this person knows what to avoid, right? But, if this person now knows to avoid it, and does, they how can it be that the future has information of the historical avent anyway? Does that mean that future has now been rewritten, or that somehow the event will still occur? Can you change the future? Create and alternate one? If someone travels from the future into the past, that means that the future exists at that point as well as the relative past, otherwise, the person who would have travelled would not exist. This is because, if you travel backwards in time, you travel backwards with time. Since people get older as time moves forward, would it not be logical to assume that they would get younger as time moved backwards? Their thoughts would therefore regress as well, making them forget their initial quest in the first place. They would de-evolve until they, personally, did not exist physically, making the mission impossible. The only way in which someone would not physically regress would be if we somehow assume that we move "forward" into the past, aging normally through our journey.
If this is so, then should we assume that when we are in physical motion, our biology can only move forward? If time is an orderly and stream-like continuum, then time travel would be impossible for the simple fact that since time regresses, so would all things from within time. If, however, somehow, all times simultaneously coexist, then it would make time travel possible, as well as explain the ability to "see the future." Because, unless the future has happened or is happening somewhere, how can you "see" it? If somehow time travel occured, then those from the future have altered the past. This also would lead us to believe that alternate versions of a time period can be created. If the one past had been altered, the futher of the travelers would be different, perhaps not allowing for the same people or places to exist...