How many times have you been reading a book or watching a movie and the bad give the hero a break, whether intentionally or by accident or they do somthing too stupid to be believable?
If the villain relies on being dumb for the hero to succeed there is a major flaw in the character.
A villain who stops to explain their diabolical plans and in doing so gives the hero a means of escape, just doesn't cut it.
Although it might have worked in the spy thriller's written by Ian Fleming and the films of that genre, in Fantasy we need to have antagonists who are consistant in their villainy. They need motive and they also need to be worthy opponents to a worthwhile hero. Don't sell them out for less.