The villain monologue is perhaps the best example of "all in the execution." I won't name any names, but there's a certain mystery series whose villains, while calm, collected, and savvy throughout the story, lose their minds in the endgame and confess their deeds to the police in a wild-eyed rant... because if they didn't, the hero would never be able to convict them. I have to admit, I think this is cheating. At least I feel cheated.
On the other hand, executed well, villain monologues can be the best part of the book. They can give the reader a hint of the villain's personality in a story told from the hero's perspective; maybe Voldemort breaks out the football playbook, but Aunt Petunia's monologue to Hagrid, when she explains why she's so angry at Lily, answered some questions about her character without distracting from higher priority problems. They're great for putting a last-minute twist on a plan the hero thinks he knows; like in the Fablehaven series, where Graulas combines 'villain monologue' with 'unreliable narrator' to such wicked effect. Plus, some genres practically require them. The audience may joke about how real bad people don't do that, but in the end, they expect them to be there and feel disappointed if they aren't.
So, I'd say, if you want to write a monologue, write a monologue. They are not inherently bad, and if you don't like it, you can always take it out later.