The Flea

I dont understand how the flea could take life from anything.  I think that the flea might be a person that is cheating on the authors wife.  So i think that the whole poem is about adultery.  In the beginning i think that he his questioning whether or not his wife is actually doing what he is thinking.   Then in the middle of the poem when he talks with the three, he finds out that there are three people in the relationship and he becomes depressed in a way. then he talks about the purpled thy nail in blood of innocence he is saying, how is this fair to me? i didnt do anything.