How to die by falling?

Assuming you're talking about rock climbing, the one thing I know for for sure is your Sim can only die by falling from the Croft Icefall, so you have to go on a mountain climbing expedition to get there. Now, don't quote me on this, because I've never tried it out--but my theory is as follows: the best way to get your sim to fall is to not do any rock climbing prep--no gear, no chalked hands, no nothing. Have them be at the lowest level possible for the climb (I think it's level six to even attempt Croft--before that you can only inspect climbing conditions, which isn't very life-threatening.) Have them perform the climb in inclement weather and bad climbing conditions. Have them climb when they're exhausted and/or their mood is low.

Again, only theories based on other gameplay activities like swimming. Never actually tried it out. My sims always seem to survive the Croft Icefall, which doesn't always play into the narrative I want, lucky stiffs. But I haven't tried all the methods yet. Good luck! (Did I just wish you luck on your attempts to off your sim? Yes. Yes I did.)