So your villain is scheming and he comes up with some sort of awesome plan, but the plan has a pretty big hole in it, and you run it by your sounding board people and they point out the hole and how easy it would be to fix and you think about the fix and there’s a problem:
The fix is totally out of character for your villain.
What do you do? Your villain shouldn’t go ahead with a bad plan, right? I mean, your whole story suffers if the heroes defeat him because he’s a moron. Do you have to completely revamp your villain?
When I watch the movies, I hate Voldemort as a villain because of what I perceive as major incompetence. But when I read the books, even on the same issues, I don’t despise him. The main example is in his choice of horcruxes.
There’s a common complaint concerning them. Namely, why the flip would you pick highly iconic treasures to be your horcruxes! Why would you then hide them in personally significant places? You want them not to be found? Pick seven indistinguishable pebbles and drop them each in a different ocean or rockbed or somesuch! Making your lifelines possible to trace is a question of competency.
But reading the books, I’m not sure I’d actually advocate so strongly for some competency changes because personality wise, of course Voldemort isn’t going to consign something so precious as his own soul into a stupid, disingenuous pebble. I get the need to have trophies and shinies and feel important – and Voldemort is way more prideful than me. We get a good look at who Voldemort is as a person by what he sticks his soul in and it’s actually pretty reasonable for the protags to get to know Voldemort as a person and what was important to him to figure out what were probably his horcruxes. This is a situation where pride could be argued to have led to his downfall, but it’s not like he had all his horcruxes gathered in a trophy cabinet for the heroes to find. They still had to do their research and figure out what his brand of pride meant and then hunt the dang things down and figure out how to destroy them – and the difficulty in destroying them gives Voldemort plausible reason to be prideful enough to think that they wouldn’t be destroyed in the first place.
I also could take issue with his excessive use of Avada Kedavera. You know, if you could just chill on that spell, Harry might’ve been dead by now. But another essential piece of Voldemort’s character is that he thinks muggles are total losers who deserve to be ruled by the powerful wizards. So yeah, of course he’s going to use the one specific to wizards way to kill people – precious, beloved magic that makes him so much better than muggles.
I will still call out his insistence on using a spell that backfired horrendously on him to kill the same target years later, though.
Consider in contrast the My Little Pony movie’s Storm King.
I’ve griped about him before, but just a quick reminder: he reneges on a promise to a minion and perishes for it. The movie presents it as “just his personality”, but I call it total incompetence. We haven’t seen enough of the Storm King for it to be a personality congruence thing, there were other options with the same end result that would have added real personality depth, and ultimately it’s just normal pushing of the friendship and magic aspects of the show in a sloppy fashion. In this case, the personality choice has no depth whatsoever and either needed to be edited or written out altogether. The personality move was more a convenience of plot than actual personality.
There’s also Nettlebrand, from Cornelia Funke’s Dragon Rider.
I don’t find Nettlebrand to be a particularly effectual villain; he’s just an angry golden dragon who wants to eat other dragons who is so busy with his rage and desire to hunt, he’s not necessarily enjoyable at all. That said, the heroes still defeat him far more than he defeats himself, and none of his abrasive personality directly causes his death, even if it plays a role here and there.
The important thing to remember here is that villains are people and people make mistakes, but it all goes back to whether or not the villain defeats themselves. Sometimes a story calls for an airtight mastermine of villainy; most stories won’t necessarily, so if your protags defeat the villain with their own cleverness and strength, perhaps using tools of the villain’s flaws, then you’re fine ditching the obvious fix. Just make sure that your villain was developed well enough that the audience can feel comfortable with his poor decisions and flaws, and consider tasteful lampshading if necessary.