It's a nasty problem. Ultimately those responsible are those consuming rhino horn; they're paying for poachers to murder rangers and render a species extinct.
Interesting how people ascribe responsibility in different ways to thigs. Ultimately, those responsible are those killing the animals, thereby providing supply to a market for it.
It might be useful to talk about proximate and ultimate causes in that case[1]. Yes, the poacher pulls the trigger, but if the horn wasn't worth anything it, that wouldn't have happened.