Morality of having your whole dynasty slain out of paranoia

I'm building a world and one of the main characters is a prince named Sergei Giada, who's the second son of King Berthold Giada of The Vinvino Kingdom, he survives an assassination attempt which leaves his legs crippled, his right eye blind, and many parts of his body damaged.

He later learns that the assassination was plotted by his own mother(Queen Marie Giada) and his older brother(Prince Alexander Giada) who wanted Sergei gone so that Alexander wouldn't have any pretenders when he takes the throne from King Berthold, Sergei and Alexander's father.

I was planning for Sergei to become king through scheming and to have the rest of his entire dynasty (except for the ones who weren't in the Kingdom of Vinvino) thrown into dungeons or be slain.

I believe it's a bad thing, but I want to see the opinions of other people as I don't have many people I can discuss this with irl.

I hope this post doesn't go against any rules whatsoever, and I also hope my English wasn't bad.