I had a peppermint that went completely insane. As a result, I will never get another one of this kind of shrimp.
Before I was finally able to trap it and get it out of my tank, I actually observed it killing and consuming the following (not to mention the things that went missing when I wasn't looking!).
One peppermint shrimp
One skunk cleaner
One blue linkia star
Several snails of different species and sizes
After it killed the $20 cleaner shrimp and the blue linkia that I had for a long time (and blue linkias are hard to find and keep in good healthy condition), I decided it was time to get it the hell out of my tank.
Before the peanut gallery says my killer shrimp was a camel back and not a peppermint, I know the difference and the two species don't look so similar that I would confuse them. I posted a picture of my shrimp here back when this happened and it was indeed a peppermint shrimp.
When this happened to me, I searched for similar stories and found several accounts of peppermints going berserk and becoming canibalistic.
My LFS guy says that when he has aptasia problems in his display tanks, he tosses in a group of peppermints. He says that in his experience, they often cannibalize eachother and do damage to other animals.