you have to see the big picture if he just killed a branch member inside the families domain there would be a full investigation, not only did every single kid watch them leave together. it would come to light he was the last to see him. they would find no signs of an intruder or assassin making the family belive it was a inside job. it would make it extremely hard for him to hide the fact that he is still injured, him hiding this facts sets up the story to make it easier for him to take his revenge on those who bullied him. and a branch member isn't completely useless, making him a servant brings added value and purpose to the character design. understanding the premise and the value of things is basic reading.

See this! I just gifted the story: Balloon