It's important when you're writing a high stakes story to have a convincing villain. Therefore, the villain should behave as any logically minded but terribly evil person would. If he has the main character in his power, he should either attempt to kill the main character immediately or have a good reason for not doing so. In my opinion, there is typically only one reason that is good enough: the villain should be afraid of the main character's growing strength. This is the only reason that will prove satisfying to a reader. In other words, the villain might hold back because he is afraid of getting hurt. And then he should have his minions severely weaken the main character so that he, the ultimate villain, has a better chance of finishing him off on the first attempt.
In my opinion, villains shouldn't monologue. But unfortunately, authors do need to have an interesting way of doing their exposition. Sometimes having the villain chat for a couple of paragraphs is a good way to reveal his plans and raise the stakes. But this should never happen in the presence of the hero, unless the hero has fooled the villain into thinking he has sided with him. If it does happen in the presence of the captured hero, it only slows down the tension. The best villainous monologues will happen in third person narration, when the hero is in a different location from the villain, and the villain is coming up with some nefarious plan elsewhere. Then, once he's finished talking about it - which shouldn't take long - he simply needs to get up and do it. There shouldn't be any delays.
Again, there are exceptions to this rule. If you want a lesser villain to look incompetent, you may want him to delay his plans. But the Top Dog Villain must command complete respect in the story. If he is not powerful enough and smart enough, it will lessen the glory that we accord to the main character, who must defeat him. And if it lessens the glory accorded the main character, it lessens overall reader satisfaction.
While Russell and I were chatting about this, he mentioned a website called "If I were an Evil Overlord", which is a great summary of the main tropes authors can fall into when depicting villains. Full credit for the discovery of this website goes to Russell Proctor in this blog post.
Here are some of my personal favourites on the long list of tropes to avoid:
One of my advisors will be an average five-year-old child. Any flaws in my plan that he is able to spot will be corrected before implementation.
I will not gloat over my enemies' predicament before killing them.
The artifact which is the source of my power will not be kept on the Mountain of Despair beyond the River of Fire guarded by the Dragons of Eternity. It will be in my safe-deposit box. The same applies to the object which is my one weakness.
When I employ people as advisors, I will occasionally listen to their advice.
I will maintain a realistic assessment of my strengths and weaknesses. Even though this takes some of the fun out of the job, at least I will never utter the line "No, this cannot be! I AM INVINCIBLE!!!" (After that, death is usually instantaneous.)
Despite its proven stress-relieving effect, I will not indulge in maniacal laughter. When so occupied, it's too easy to miss unexpected developments that a more attentive individual could adjust to accordingly.
If it becomes necessary to escape, I will never stop to pose dramatically and toss off a one-liner.
If the hero runs up to my roof, I will not run up after him and struggle with him in an attempt to push him over the edge. I will also not engage him at the edge of a cliff. (In the middle of a rope-bridge over a river of molten lava is not even worth considering.)
And here's the link to the website, so you can peruse it yourself.
Lastly, here is Russell Proctor's Amazon page, in case you want to find out more about him as an author.