After his brother died he had his 2 young nephews declared bastards and took the throne.
The boys disappeared soon after.
He was the last English King to die in battle.
That stuff is tame compared to some things in Chinese and Korean history
...Although Yi Bang-won, Taejo’s fifth son by Queen Sineui, had contributed most to assisting his father’s rise to power, the prime minister Jeong Do-jeon and Nam Eun used their influence on King Taejo to name his eighth son....
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....Both sides were well aware of each other’s great animosity and were getting ready to strike first. After the sudden death of Queen Sindeok, and while King Taejo was still in mourning for his second wife, Yi Bang-won struck first by raiding the palace and killed Jeong Do-jeon and his supporters as well as Queen Sindeok’s two sons (his half-brothers) including the crown prince in 1398....
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They were literally slaughtering their own family members for power. There was another instance where a King slaughtered everyone in his and his wife’s family who could possibly challenge him for the throne. He didn’t even wait for anyone to get that idea.