Yea, and I had murdered many of his children
Wait, what? Alma was a murderer before he repented? No, see what he says next?
or rather led them away unto destruction
So no, he did not kill their bodies. But he did lead them away from the truth. He led them off of the path. He led them towards hell. He removed any righteousness in them. He killed their soul.
And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.
Of course, then he repented and try to fix what he did.
And now it came to pass that Alma began from this time forward to teach the people, and those who were with Alma at the time the angel appeared unto them, traveling round about through all the land, publishing to all the people the things which they had heard and seen, and preaching the word of God in much tribulation, being greatly persecuted by those who were unbelievers, being smitten by many of them.
But notwithstanding all this, they did impart much consolation to the church, confirming their faith, and exhorting them with long-suffering and much travail to keep the commandments of God.