Rhetorical: What is Alma's first "evidence" that one might have continued to sing the song of redeeming love? Alma's first question is "have ye walked, keeping yourselves blameless…" What Alma is telling his congregation is that the commitment to the gospel must be greater than words alone. The commitment must be to live according to the dictates of the gospel (which certainly do not include the murders nor "all manner of wickedness" of which Alma has accused some of them!). Alma then moves to a deeper plane of righteousness. Even should they believe that they had been walking in the ways of God, have they "been sufficiently humble?" Why does Alma specifically bring humility in to the picture?
While it is true that humility is an important attribute to cultivate, Alma's use of the term here has some very specific connotations that will become clearer as he develops his argument. In this particular case, Alma is moving from the egregious sins against the laws of God (and not so incidentally which would also disrupt society) and into those which may appear smaller, but which nevertheless are at the heart of the current social unrest.
What Alma has done is chastise first those who are blatantly causing strife. He will now press home on those who perhaps are inadvertently creating the strife through some of their actions that they might not even consider sins. These specific actions, according to Alma, will grow out of their lack of sufficient humility.