Is God really Merciful according to the Bible?

1st Samuel 15, Saul disobeys God but the Bible says that God is Merciful also He commanded that "Thou shall not kill". Why did He ask Saul to kill all the Amalekite people without sparing their livestock too. I mean that really sounds Wicked to me asides it contradicting the commandment. How is God Merciful? If He's sending Israel to kill her enemies when Jesus commands we love and pray for our enemies or for not stopping Jephtah from sacrificing his daughter(a ram was provided in the case of Isaac). How is this bloodbath Mercy or rather How is He still a Merciful God? > **P.S.** An in depth answer would be appreciated