Because despite believing the act is wrong, I just want to do it or feel that it is the best of all the crappy options I have available at the time.
But wouldn't it be easier to say "the act is neutral but obtains a positive or negative value in a certain context"?
By adding a negative value to the act from the very beginning, you'r locked and are forced to violate your logic-value system in order to carry out the act.