Causality violation is a philosophical and ethical nightmare.
If you go into the past and change history, you may be committing an entire timeline full of people to oblivion. Everything that happens after the change will cease to exist or be confined to an inaccessible limbo; after changing his past, Marty McFly might start to fade away, or would he simply blink out of existence? Either way, he's gone.
If the original timeline continues to exist, then maybe it is no longer possible to go there- but if you can go there, you will see that your temporal meddling has achieved precisely nothing for the inhabitants of that world.
On top of that, you must accept responsibility for any changes you might make - if you go back in time and successfully change history, you are morally responsible for any new events that occur in the altered timeline - which could be full of unintended consequences and may even be worse than the original world. If you have seen The Butterfly Effect you will understand the problem.
Most current theories suggest that causality violation is forbidden by the universe - and for good reason.