Karasuma wants to be a cool hero, but for some reason can't. (Maybe the green blood on Eta's face was Karasuma's, and it's the reason why Karasuma can't be a "cool" hero?) I figure he might have specifically killed this villain because he would've revealed Eta's identity otherwise.Basically a "villain" who tries anything bad but ends up doing good vs a "hero" who does what he thinks it's good but doesn't know it's evil
Well, it's also the perfect power for assassination, and a way to easily kill Eta despite his nigh-invincibility when transformed.Karasuma's power being invisibility is a great indicator of what he is. He wants to be a hero, but he doesn't want to be seen as lame/cringe by his peers.
So he got the best power for the purpose of not being seen.