Ok, after I've reported this, I had been able to kill one elder: my butler. Everyone else seems immune to them. I also noticed the description of the arrangements when given as gifts wouldn't read "scented with Death Flower" as it does while in inventory. I'm trying hard, I've tried to kill them while having a friendly relationship, an unfriendly relationship, tried several locations and always always try with Sims I haven't created, how do you call them? NPCs?. I'm sure it will work with my Sims, though I'm not risking it.
Please fix it.
That's mainly why I bought this (I'm lying, I didn't know it could be done) but it added a super plus and super joy to my playgame ♥
