The lady wasn't trampled, she was kicked to death according to one of the news articles. There's a difference, in my mind, between being knocked down by a horse fleeing/bolting and being trampled in the process, versus being kicked at repeatedly until you sustain fatal injuries.
That having been said, I am sure that it was both an emotional response by the family members and in an abundance of caution, supported by law enforcement. Given that the horse had kicked her before, and this event was another kicking episode, to sell/give away this horse knowing its history, there would be clear liability if anyone ever got injured again. It would be just like a dog that bites being given 2 chances, right, and then it has to be put down.