All the characters in the story have to be true to themselves. Their actions in response to a given situation have to be what they’d actually do, not what’s convenient for the plot. That applies to the bad guys just as much as the heroes. Well, of course it does. Right?
Yes, but. This is one of those rules not to be followed off a cliff.
In our case, the discussion was over a matter of degree: how horribly will he treat this particular person? It wasn’t a matter of would he do the nasty thing, but would he do it more than once.
When you have questions like that, the real questions you should be asking are, what kind of story am I writing, and what can the plot survive.
What kind of story: just how cruel is your villain, and when you do justice to depicting his depraved actions does it skew the tone or the direction of your story in undesireable ways? Showing what he’s capable of is important, but once that’s established it might start to seem gratuitous to let him keep living his best life.
What can the plot survive: there’s a difference between choosing what’s convenient and avoiding what’s lethal. If the heroes are neutralized, or the object of the quest is destroyed, then the story no longer works.
Back to our situation: we’re leaning toward the once being enough. (Our bad guy will still have plenty of chances to be naughty.) We don’t want the victimized character out of play for too long, and we don’t want to make victimization thematic.
So, yes. There is such a thing as too much villainy. Don’t let your villains kill your story.