Your first bit of advice is to stop drowning yourself in 'should' or 'what if' and anything of the sort. Being repentant is fine and expected, but there comes a time when it crosses into 'beating the shit out of yourself'. And that's not productive at all.

[Ardyn shook his head, setting down the coffee mug and leaning his head on one hand. If he really had been around for untold centuries, he didn't exactly act like it.]

Yes, you more than likely should have been tried and convicted at minimum if not just outright killed. But, you're both here and alive. You're going to need to come to terms with that rather quickly, because only then can you decide what to do with that.