These behaviors have been present in oral histories of elephants for the last two thousand years or so. As far as I know, the intense hunting for tusks responsible for the endangerment of elephant species has only been present in the last few centuries. My bet is that this self-domestication has been occurring for a long time. At most, the recent selection for killing those with aggressive behavior, large tusks, would be an additional factor in this process.
"At most, the recent selection for killing those with aggressive behavior, large tusks"
Are million+ years recent?
Humans successfully hunt big animals since we exist and all the hunter societies that still exist, share the idea that the biggest trophies come from killing the most fearsome animals. And if an animal killed a tribe member, killing that specific animal comes with great respect for the hunter. So the organized hunting of elephants especially for their tusks might be a recent thing, but killing dangerous individuals is not.
Is it also possible that elephants have enough awareness to notice who among them is more likely to be murdered by humans, and start selecting partners differently?
Of course there could be sexual selection at play! I would argue though, that form of selection will take at many generations to take hold where as genocide of a species happens in a single generation. Of course, the smaller the population, the faster sexual selection will take effect but it might be hard to disambiguate with other fitness factors.
Fwiw, a very similar thing happens during selection for bacterial resistance. You eliminate all the cells susceptible leaving a population of resistant cells. This population then grows to replace the previous generation. You could analogize this with the elephants. Small tusk and docile elephants have a "resistance" to being murdered by poachers. The poachers eliminate all the others and suddenly the population seems domesticated.
