Ask whoever you like (or don't) this question on the very basis that humans are evil by design: "What's the evilest thing you ever did?"
Chances are they'll take some time before coming up with an immediate answer. Besides, you aren't asking somebody their age so it's alright if they take their sweet time to come up with an answer. It doesn't really matter whether they are being candid or making shit up. But it'd be really sad if a person doesn't have an answer. Worse would be the person who'd be audacious enough to claim that s/he doesn't remember anymore. After all, isn't forgetting the evilest thing you ever did simply the evilest thing you ever did?