Debate Question: In murdering a child whom he "feared" would "overburden his parents with disbelief & transgression," did Al-Khidr set a proper example for Muslims to follow?

If a brother murders his sister, a sister who has DEFINITELY BECOME A DISBELIEVER and who thereby has "overburdened his parents with disbelief," has this brother, guided by Al-Khidr, committed a sin?