I chopped off a penis with an axe a few times in past dreams. I had this dream after being raped so for me it made sense to dream this. Maybe look back in your past and see if you feel any anger towards penises, or men. Freud would say you're just envious that you don't have one.

Dreams are so complex it's hard to pinpoint a reason behind them. I try to go with the feeling first, then look for symbolism.

Were you angry cutting off this man's penis with an axe? Or maybe you were serene while doing it.
AFter I identify a feeling, I think about my present life and where I am having those feelings in the waking world. Then I try and make connections.

Either way, It doesn't sound like a pleasant dream.