I always thought it was pretty straight forward. You can never go back in time with the intent of doing something specific to alter it, even if you're sent by someone else to unwitting do their bidding.

As soon as there is an intent to "correct" time then taking action on that intent cancels the original cause of concern so that now, there's no reason to go back.

"I'm going back in time to kill Hitler" (goes back in time and kills Hitler). Now, he didn't do all those horrible things so there's no reason to go back so I don't go back when that same second comes around again and history is restored to it's original configuration. (or something earth shattering and horrible happens)

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250