Ohh...
Look at the odd jobs on the phone. If you have Island Paradise there should come an option to take a job to go to the beaches and kick down other sims sandcastles.
Place dollhouses in the park, wait until the kiddos come over and then break them.
These are not super-evil but a nice little stuff to do.
I have one evil sim that I play regularly and she does NPC-management. I have a huge rotational save with multiple families. Since I want the family trees to stay intact instead of culled I let a evil sim manage the death part. All elders except for my main household sims are moved into her household. They get dinner and a present, a flower arrangement scented with deathflowers, which makes them keel over ASAP.
Hmm... If your evil sim is an vampire you could get risky vampirism. If one of the dinner dates turn into a vampire they get taken into the household. Those who do not please their new maker get kicked out of the house at about lunch time. A small huddle of graves near the front door might keep new arrivals on their toes.