Could doing good affect one's past as well as the future?
As an example or retroactive karma, someone has a bad childhood and at the age of 30 robs/murders another person. Most would say that the person's hard life led to his robbing and murdering. However maybe the bad karma of harming another person at 30 sent bad vibes back to his childhood and negatively influenced his past.
(This could be related to the idea that time does not exist, only the present exists and the past and future are "here and now")