This is not a 'Is time travel possible?' thread. With that said.

Imagine this scenario.
Its 2357, and you invent a time machine. You go back to the year 2003.

Now, the question is, could you really effect the future?
If you kill someone, were you meant to do it, as in they died in 2003, you just went back in time and you were the one who killed them. Or say you want to kill the president, so you try to kill him, say it was yesterday. But because he is living today, it was impossible to kill him.
Basically.
If you went back in time, do you really think you could affect the future? Why or why not?