Any type of Protagonist that's a Mad Scientist Supervillain. I'd prefer long stories, but short ones are okay too. Also, I also prefer lonely without teams MC type, but it's okay with that too.
Mc is a medical student, living through a sort of (?) apocalypse, where all sorts of creatures/monsters appear. He gets missions from a system, and dissects these creatures/people. Not super "mad" scientist, but very anti-hero vibes, so I thought it could fit very creepy too!
Thanks. Anti-hero MC's are my type, so I'll read it to see if it suits my tastes. Haven't read much medical/doctor type novels, but I'll see.
Ouroboros Record ~Circus of Oubeniel~ The MC died and got reincarnated to another world. After experiencing death, he became very scared of death. So he learned alchemy and got very obsessed with immortality. He would really do anything to gain immortality including human experimentation. Does that count as a mad person though? Because my first impression of him is actually a smart, cold, and manipulative person.
https://www.royalroad.com/fiction/36804/the-devils-foundry - mad scientist super villain, along with the hero, are transported to a magic world after being thwarted by a super hero. The villain needs to uplift the locals to a point she can figure out how to make a portal to go back home.
one on royalroad called vaudevillian, its about a vr game where everyone gets to be hero's or villains, and the mc decides to be a comedic old time villian with top hat and mustache and does a bunch of funny evil things, its prob not what you're asking for but its kinda fun
The Mech Touch. The protagonist really takes the cake for protagonists who i have mixed feelings for yet attached to. He is a mech designer, a mix between an scientist and artist, and engineer and producer. He will research, discover, and do crazy experiments moving forward. It's at 2200 chapters and the only time I got bored was an arc from chapter 600-900ish, other than that, it picked up and became better afterwards because that arc was more of a build-up.
Soul of Negary - The MC isn't necessarily evil, he's not really mad, he's just...rational. He can definitely play the role of a final boss in a novel. This fits your description most. Experimental Log of the Crazy Lich - The MC is crazy. I Have a Super USB Drive - Not mad, but not too humane either. Way of the Devil - The MC is just basically convincing himself that he's still a human at this point. Cultural Invasion To Isekai - This barely counts, the Mc is an engineer, but I just like to add it to complete the list.