Okay so this upcoming film Acolytes...
Has a pretty spiffy premise. My kind of premise. Socially irresponsible!

I'm going to try to keep this at a cool speculation spoiler level of about .5 milligrams per second.
The premise is that these kids discover the identity of a serial killer, and twist his arm into helping them get revenge on a childhood bully-- at which point something goes wrong. Wronger, anyway.
So just as a sort of dip stick measurement of the collective insanity out there, I'm inviting people to comment with their own baggage/damage on how you'd best abuse a serial killer's services?
No need to name names here, as we don't want any bodies turning up-- but let's all just pretend we have no conscience here-- If you could blackmail a serial killer, what would you have them do-- and how would you protect yourself?
Here's some ideas to help percolate the pot:
1. Laundry, or dishes, other menial tasks. But this doesn't really take advantage of a professional murderer's innate abilities. On the other hand, they'd probably have their own latex gloves-- but depending on the 'school' of maniac we're dealing with here, they might also substitute dish soap for ocular fluids. OR... clean everything pristine, but then chop you up for dirtying them again.
2. Revenge... is so over saturated at the moment. Let's see what the actual movie does on this topic.
3. Pest control; like bugs and rats and stuff? Let's see them rig mini nooses and SAW-like games on fire cracker budgets for some vermin. Go back to those childhood roots, and put em to some positive use!
4. Return late library books. Just make sure to exclude any copies of "The Catcher in the Rye" to avoid red flags.
5. Negotiate the conditions of the warranty on new electronics at pushy big-time retailers. Seriously. Try selling these guys air, and see if they don't snag a killer deal.
6. Kill that annoying boss you can't pass in a video game. Okay, so this doesn't account for motor skills, but more appealing to the psychotic determination and narcissism that only a cold blooded killer could possess.
7. Get them to give you a really amazing massage. Okay, so this one's pushing it a little bit, putting your neck on the proverbial chopping block-- but I'm assuming as a member of the midnight underground, you have the nerve for it. Seriously.. if they're really good at killing people they should know all about pressure points and human anatomy. There's got to be an up side to that.
8. Navigate the automated maze of a major telephone company's costumer service hot lines, to argue over your phone bill. I'm talking touch tone. Only problem here, is the red flag around the voice disguiser when they finally ask to speak with the manager.
9. Use them as a babysitting/parenting deterrent... Like that old "Far Side" comic. Lock them up in the basement with a bunch of crazy power tools, and threaten the kids you're responsible for whenever they're bad. Oh, you wanna see REALLY bad? You go spend some quality time with the bad man! (Okay so this one's pretty bad, but... I mean how responsible are you anyway, messing around with a serial killer?)
10. Bounce ideas for screenplays off of them, to stay fresh, original and realistic! New franchise here we come!