English is not designed to describe time travel. In general, if you are writing from the perspective of a character (including third person subjective), use tenses that make sense in terms of that character's perception of time: if something happened in the world's future and the character's past, use past. If you are writing in third person objective, use future to describe the same thing. But there are no firm rules: do whatever seems to make the most sense.

I don't know how to answer your question properly. Here are some ideas though. The mad scientist, Albert, built a machine to allow him to travel through time. One day, he went back in time. While there, he killed his father. Doing so caused a paradox - was Albert still alive after that incident? He had killed his father, which meant that he should not have ever been born, nor should his son have ever been born. If he had never been born though, he would never have been able to build the time machine and travel back in time to kill his father. or Albert contemplated life for a while. "Even though I do not have official accreditation as a scientist, I still consider myself one", he said to himself. "My peers don't understand me, in fact some have called me a mad scientist! That doesn't bother me, because I have created one of the most amazing machines ever invented - a time machine! wuahaha". With that, Albert switched on the machine, jumped in, closed the door and was immediately transported back 50 years into the past. He opened the door, and found himself in a paddock. After walking for nearly three miles, he found his way into town. It didn't take long to find his way to the garage where his father worked. {{{ he kills his father, etc }}}. It was all over, Albert thought to himself. At any moment he should cease to exist. Seconds, minutes and hours passed by, but Albert was still alive. Many thoughts entered his mind. He wondered how he was still alive, here in the past, when he shouldn't exist at all! If he were to travel back to the future, what would the world look like? What would become of his son? or Stuart asked Alice, "where is Albert? I haven't seen him". Alice replied softly, "he has gone back in time!". Stuart was shocked and asked, "Are you sure? What is he doing in the past? Will he be able to return safely to our present time? I hope he doesn't do anything stupid in the past! He could ruin everything for us here in the present!".

Here's how I might say that. I went to the past (to a time before I was born) and killed my father. Because I killed my father I should never have been born. Therefore I should not exist in the present. Since I should never have existed, I should never have been able to help create a son. Therefore my son should not currently exist.