I read Don Paolo as someone who claims he’s the Evil Villain of the story but he’s actually really bad at being evil. I’m not even convinced he thought far enough ahead in his plans to go “wait, I could legitimately kill someone here.” (Hence his heel-face turn in UF, which works just fine even though there’s no real buildup.)
So… he’s visibly a little strange, his inventions are cool but weird and sometimes scary and have probably never been properly appreciated, and he fixates on the most obvious outlet: the handsome, gentlemanly, and widely admired professor who (in his mind) stole his crush.
(Honestly, I’m amazed that Descole never thought to hire the guy.)
So… yeah, his canon motivations are pretty weak and were probably thrown in at the last minute. And yeah, he deserved a more developed backstory. But I agree that he’s more of a comic relief villain, so I’m not horribly bothered by it.