Absalom killed (or had his servants kill) his half-brother, Ammon, for raping his sister Tamar.
See 2 Samuel 139 for the details.
Now Absalom had commanded his servants, saying, Mark ye now when Amnon's heart is merry with wine, and when I say unto you, Smite Amnon; then kill him.
And the servants of Absalom did unto Amnon as Absalom had commanded. 2 Samuel 13:13-29