It's about finding the sweet spot for being murder-y. You want to be just murder-y enough to be interesting and mysterious, but no so much that there's actual fear for their lives. OTOH, being not murder-y at all is boring and unfuckable. The dude with the man bun writing poetry about birds is not getting the date in the first place. Meanwhile, "probably not a murderer but I can't rule it out" is heading back to their place for a nightcap.