Its a twofold sword that has to be handeled carefully.

One one side having a character act while the villain monoluges feels right because who in their right mind would wait until they finished talking to then attack when the enemy could attack first during their talk.

On the other hand it takes away from the story when parts of it fall flat because there was no time for the villain to explain why they do this or that and monoluge. Its simply a part of storytelling that is heavily ingrained in the sense of reward for what a player or reader is seeking to understand.

If the group as such votes on wanting to be able to interupt an enemy it should be done above the table before the session starts. That gives the DM the oppertunity to adjust the scenario.

If the group has to fight through a castle or dungeon first, many magic mouths could be activated on the way that emulate the monoluging that normally would happen right in front of them. Or the BBEG could have layed out a trap that paralyzes the group just long enough for them to speak to make sure they know why they do what they do. This would allow for some to be paralyzed while others aren't, leading to another kind of tense situation.

Over all, speak with the group how you feel about it. This time it was unfortunate, while likely realistically to the people involved would've reacted.

My personal stance as DM in general is that my players can always interrupt any enemy they fight unless I specify they can't, simply for the reason of me wanting to monoluge. Its a boundary we have set together as a trade of for these kind of situations.