is time travel ethical?

time travel will exist as science fiction for now, but i'd like to hear your thoughts on it :)
do you think it's ethical to travel to the past and future and rewrite history (for own gain?) or foresee what will happen?

i have had mixed feelings on this for a long while. i think the past was the past and people shouldnt meddle in it to change what's happening now. also, the future should remain a mystery because no human is supposed to know whats ahead....

what do you think?