Rhetorical: Alma continues to press the sins of his congregation. His theme has been the possibility of redemption through the Atoning Messiah, but his direct focus has been on the sins of his congregation. In verse 23, those sins are exposed as gravely serious. We need not suppose that Alma is exaggerating for emphasis here. In the context of the times and the sermon, it would appear that there are those in his congregation whom he is justly accusing of "all manner of wickedness" including that some are "murderers."
The contentions in Zarahemla are not limited to church-members/non-church-members. The contentions and causes of division have arisen within the church also, and Alma is combating those contentions directly. In this particular case, Alma begins with the declaration that there are gross sinners among the congregation claiming to be the church. Perhaps such people have justified themselves in their actions because they were members of the church. Perhaps they thought that having been baptized once that they were somehow immune from sin. Alma needs to impress upon them that while the baptism into the church begins the journey, one is required to continue to monitor his actions and have them conform to the gospel.
It is in this context that he identifies the sins of the sinners, and specifically asks them how they will feel standing before the bar of God with those sins. Perhaps they had thought them erased, but Alma is telling them that they are not. Those who commit such acts (whatever "all manner of wickedness" might be) and are unrepentant, they will have to face God in that condition.
Social: The divisions among the people of Zarahemla are apparently not simply ones of theological differences. The passions behind the divisions have apparently been sufficient that Alma may justly accuse members of his congregation of murder. While it is certain that the anti-church forces would have been equally as forceful, and perhaps murderous, the body of the church was not without culpability. In some ways, this is eerily prescient of some of the problems between the early modern church and its enemies. While the enemies were culpable of sins, those inside the church were not always wholly innocent either.