What purpose vindictive behavior serve?

Science Answer: We have done most of our evolution in a prehistoric setting where social interaction only served to advance the interests of an individual. Vindictive behavior serves to get to the top of a social group by force.

Religion Answer: Sin is a natural part of life that comes from free will. Vindictive behavior occurs because of our free will and the fact that we are naturally 'evil' to a certain extent.

Really the two answers do not conflict greatly.