I've killed a lot of sims for the Black Widow Challenge and I've gotta say my favorite way to kill sims is either drowning or hysteria. I've tried starving but it takes soooo long omg. But what meth...
I found that dressing them "wrong" and sending them outside on a hot or freezing day is rather quick. And I think it looks a tad better in the family tree if they freeze to death rather than starve...