However, the sin next to murder isn't non-marital sex. The sin next to murder is using one's power and position to draw people away from God.
Alma speaks to his son Corianton beginning in chapter 39. He rebukes Corianton sharply, listing his wrongs. Alma's chief complaint against Corianton was that he forsook the ministry. While he was forsaking the ministry, he visited a prostitute, which is what most people latch onto, but from the text, Alma is much more worked up over Corianton setting a bad example and leading people away from the word of God by his behavior.