Apart from the inherent meanness that we can feel in the actions of the chief judge, there are other aspects that may be coming to play here. The chief judge strikes Alma and Amulek. Why does he do this? We may claim pure devilishness for the chief judge, but it is also possible that he is following a cultural custom toward important captives. The Maya kept important captives alive, but did not treat them well, and in fact tortured them periodically. Thus we may be seeing a chief judge treating Alma and Amulek as important captives.
The second point is that the chief judge has clearly connected the burning of the believers with the putative religious heresy of Alma and Amulek. He sarcastically asks if they will be able to preach that the Ammonihahites will end up in a lake of fire and brimstone now that they have seen their believers perish in a lake of fire. This is no coincidental image. There was a religious motivation in the taking of their lives, and likely a religious motivation in selecting the mode of death. That the chief judge saw the future punishment of the wicked in a lake of fire and brimstone as analogous to this sacrifice of human life suggests that he may have also seen the verses about the burning of the wicked in the time of the Triumphant Messiah as a similar religious connection.