Full Betazoids are fully telepathic, not just empathic. Knowing all the heinous shit people are thinking but don't say outright can't be good for their sense of empathy!

Serial killer? No. I can't even say he was a psychopath really. But there was a dude, Suder, who had insane violent tendencies but was aware of them and could (generally) control them. He knew it was unacceptable so he looked for acceptable outlets to get it out. Working with the Maquis was one of them because who cares about dead Cardassians? But after being stuck in the Delta quadrant, he had zero outlets for it. Was about a year of it that he couldn't handle it anymore and gave into an urge and just killed a Voyager crewman, but only the one. Tuvok mind melded with him to figure out wtf was going on in his head and Tuvok nearly went insane for his troubles. But they spent a while working together and Suder had everything completely under control. Working on plants and shit. But the Kazon attacked and he had to go a-murderin' again. First dude he kills actually gives him a full blown panic attack and he has to be basically talked into killing other Kazon. He then gets his shit together, kills a fuckload of Kazon and manages to take the Voyager back but dies in the process.