His Death Was Elena's Fault (Spoiler)

If she hadn't stepped in to prevent Diego from killing Rafael it wouldn't have given Rafael the chance to gain the upper hand by pulling the gun and threatening her in order to get Diego to drop his sword. And in doing so Diego was ultimately shot by Rafael which was the cause of his death in the end, even though before he succumbed to his wound he was able to take out Rafael.

In fact, she basically was responsible for both of their deaths. She was the one that freed Diego and accompanied him to the mine for the final showdown. It was no great secret that Diego's ultimate goal was to kill Rafael, which is what ended up happening after all when, despite his injuries, Diego tripped the breaks on the wagon which Montero was tangled in and pulled him over the cliff edge to his death. She had to have known that by freeing Diego she was allowing him the chance to exact revenge on Rafael; inevitably one of them was going to die in a fight to the death. And given that Diego was always the greater swordsman it was inevitable that he would have bested Montero in the end.

Diego could have survived and gone on to live a happy life, reacquainted with his long lost daughter had she not interfered.
Good job Elena 😒