I feel regret

So now that the adrenaline has gone away. The anger has been quelled, I feel regret for how I killed Osric. I remember him cowering as me and fellow adventures rained blows on him. He tried to go invisible but was denied. It ended with my hands around his neck till he was no more.
He was a former knight, he had to had honor at some point. Did he deserve a trial? I dont know how to feel...

At that point in time, we could not safely secure him and bring him to a fair trial. I was the one who denied his spells, I rained spell after spell upon him. I did not attempt to stop you from killing him. I believe I was the ranking noble present for his execution, so if there is to be repercussions for his death, that falls to me. You fought to protect your friends and family.

If it helps, he wasn't Knighted for honor or for any of the usual reasons. He was Knighted so that Baliol could make good use of his teams. In exchange he got what he needed from Baliol. There was no honor involved. Osric was a man of opportunity and manipulation.

Khala Stormbringer

Echoing what Khala said. While I had a brain spike, Baliol and I had a number of conversations, one of which was about Osric. Baliol himself referred to him as a man that serves only himself.

-Ser Robert Morgan
Lupis Gemini

He was knighted by scum, so he is scum. The scunner died badly, as he deserved to. Dinnae beat yourself up o'er it. He embodied the worst qualities of those we call the Lowlanders or Outlanders- Sassenachs, in the old tongue.

Artalon,

He was offered terms for surrender - he was even offered amnesty for his crimes under the Duke, if only he would bend his talents and knowledge towards the good of the true Kingdom.

He chose instead to take advantage of Travance's kindness and used the opportunity to grasp for the power locked behind the Weavesmith's keys. When you choose to pursue that kind of power, there are only two outcomes: success, or death. Anyone walking that path knows this, and it echoes in your mind with every step you take; there is no doubt in my mind that he knew what would happen to him if he failed.

If it is the specific method by which you slew him that bothers you, there are many in this town who have learned how to find a kind of peace on the line between what is necessary and what is monstrous. You may want to explore this further by speaking with Pendarvin's own stalwart knight.