Frankenstein, a fictional mad scientist
A mad scientist was any scientist that was perceived to be mentally unstable, such as Frankenstein, from the classic Mary Shelley novel.
In 2024, an article of Featured News that was written by Noah Schloss referred to Doctor Adam Soong as a "mad scientist." (PIC: "Two of One")
In 2375, Tom Paris expressed his (temporary) desire to delete his The Adventures of Captain Proton holoprogram, saying he'd had enough of robots, mad scientists, and death rays. (VOY: "Bride of Chaotica!")
In 2399, when Doctor Altan Soong introduced himself to Jean-Luc Picard, he described himself as a "mad scientist." (PIC: "Et in Arcadia Ego, Part 1")