1. Go back in time
2. Kill someone
3. Future explodes
INCORRECT!
Depending on when and where the person you are killing takes place, the results would be different. Atleast thats what I think.
Example. You live with 2 people, not counting you. 1 of them mysteriously dies by falling off of a balcony. You (Insert random comment about time travel invention here) go back in time to see what caused this. You appear right next to your friend while he/she is on the balcony. You appearing scares the living stuff out of them and causes them to fall off the balcony, and die.
So no matter what you do, the event would still be the same because you altered the time line in the first place.
How ever, if you go back in time to a period of time where you weren't around and you influence an event, such as killing someone important, then the future would explode. :D