Not "after they were discovered", but after they were discovered AND the father beat the crap out of him AND order ppl to kill him. It was either he'll kill or be killed. (still he was able to kill the parents, and some servants too, in order to hide his crime, so maybe he could've beat the servants and run, I don't know...)
-> Although it might sound like it, I don't want to defend him^^' The story is toxic, and a fan translation isn't much helpful (esp Mosh...) when it comes to more complex stories.
I'm pretty sure it's not to "take the younger brother's life" but to prevent Dongim from getting married. That's why he locked him up, told everyone Dongim's sick to postpone the marriage. Actually he might have hoped to call it off but the bride's father was keeping being pushy and threatend he will take Dongim, so he unwillingly married the girl.
I watched the originals and plan to read it with dictionaries to figure out what this story is about, so for now I don't want to spoil (or spam?XD) with my suppositions.
It def looks like both of them are twisted though.
Oh... spoiler
actually it's all Dongim.
Kill the brother's bio family after they were discovered? Is that why he locks his brother away so he doesn't run? So the tables have turned and top is getting the.life the younger bro should have had.