From what I have read, Hinduism comes very close to explaining our actions to our karma.  I think that our actions are a sum composite of past lives and free will.  Every opportunity is given to a human being to better himself, but if his action is self-serving and shows no mindfulness of how it will affect others, then bad karma is created.  After reading about past life case histories, I have to say that it is like small children playing tag.  If indeed, we are nothing but energy, then what we say, think and do, have to be analyzed very carefully.  Since we are only human, this takes a lot of effort and a lot of restraint.  From my own experience, I have found that "reactions" to actions and the timing varies from person to person, but it appears to be quite cumulative.  I think Science has come to our aid quite a bit in order to explain the mystery of life and karma.  However, the more I learn, the more I am in awe of my own religion!  It humbles me.