Sybe07 said, “I also have seen Lal teaches that the mindset of an arahant or even a Buddha does not stop all kamma-vipaka in this life.”
Yes, that is true. But if he was not an arahant he would have spent thousands or even millions of years in niraya. so it would be correct to say he barely suffered a fraction of the kamma vipaka he was due. If his kamma was not so serious he would have probably escaped the whole thing.
As for the matter on imprisoning angulimala, the whole thing comes down to the moral question. Do we need to punish a person for the sake of punishment? or is there a more logical reason behind it?
Do we do it to teach a person a lesson and help him become better? Or are we doing it make sure that the society is safe from such criminals? Either way, there is no logical reason to punish him if he is already an arahant. He is no harm to anyone at all. I think the king took buddhas word for it and decided to give him a chance. That’s how much faith the king had in the Buddha.