Now that Ren Xuebai has ended up like this, staying alive is useless for Lin Mu. Of course, Lin Mu directly taking out Right Army also had another reason, which was somewhat to save a life for Ren Xuebai!
If Right Army were sent to the police station, it would definitely implicate Ren Xuebai. Since Ren Xuebai is already crippled, becoming a lame man, there's no need for him to spend the rest of his life in prison. After all, he is Elder Ren's biological son. It was already not easy for Elder Ren to make this decision. Both sides feel like flesh to him; try to make Elder Ren feel a bit better, and reduce the cost that Ren Xuebai has to pay.
Moreover, Right Army deserved to die, killing his own woman, Lin Mu could only give him one outcome, which is death. Therefore, no matter what, both emotionally and rationally, Right Army must die!