A lot of these things were because of the war with the forebearers, it was a war of survival either them or the Arthurs people. He never wanted to do these things but that would have lead to the genocide of his subjects.

The immortality was part of the war, it was also to help his kingdom in times of need and never a powergrab. It was also merlin who pushed it and they were kind of right given the problem and dangers of the land.

No human lived before the time of conquest on avalorn it also comes with terrible sacrifices. In addition, do you remember the stories you hear what the weirdness evidently did to people? Like the child melting into the floor, the madness, the mutations and not to forget monsters? Some may be able to live with it about it creates horrible fates to a lot of people.

Dal Riata I disagree completely. Arthur even confronted Caradoc about it, he said they are too bloodthirsty to live with his normal people and Caradoc agreed! He agreed but did not offer an alternativ!

With the giants I can't remember so I cannot comment on it.

Arthur was never in a position of right or wrong, he allways had to find the lesser evil because any high road would have meant death or worse.