Hello
I know serial killers love to keep the same pattern, but if your sim is in need of kills, he can try other methods.
The cowplant is a good way, the problem is, it must be hungry, and when it is, must be feed quickly. To die, a sim must be eaten a second time, with a status from the first time, so having at least 2 cowplant can help.
You can try to lure him in a swimmingpool, or another, a lot of action can be done "together" to ask other sims to make the same thing as yours.
In winter (and in summer?), you can use the weather (there is also a machine that can control the weather), lure the sim outside, ask to see his "winter" clothes if it's hot, ans his "summer" ones if it's cold. Try to keep the sim busy, so it doesn't go inside and doesn't change clothes by himself.
You can cook fugu, and give it to another sim. If the cooking skill of your sim is low, it could kill the eater. The problem is, the more you cook, the best your sim become, and the less luck there is that the fugu will kill...
I think these are the better option to kill a sim you don't control, but you can try other ways if you want. Could the death be with you