No, it shouldn't be a problem. Of course You cannot finish some subquests (the invitation from the Warrior's Guild) and You cannot become an assassin. The same happened to me because I killed the Rat Thief.

I lost my patience after trying to find Raskillion for 4 days in the stout mines. (4 days of switches, elevators, etc. ahhhhhh!)I couldn't wait for a walkthrough and a map that told us EXACTLY how to do the hell hole called the stout mines, so.........

I killed the king, looted everything and never found that Raskillion. Will this stop me from finishing the game?????

You will find Raskallion by taking the right fork on the mine car, then left fork. There are two switches to move around. After swimming through a small lake, you will find him. The other track fork leads to the dragon ore and lava.

The 3d map is useless in the mines. I mapped it on paper and was able to go and do the whole mine in 1 hr. It helps if you know where you're going...

I made it down all of the tracks in the stout mine, found Raskalian, pludered one of the four vaults and found the ore. I gave the 2 ore to the King (they weren't identified) and he made me one suit of armour. Why did I think he would give me two? So I killed him and his guards. I had already given them the Guild Invites and returned to town to complete that quest.

Did someone want me to kill Raskalion? Was it the king or the rat in the burned out house north of Shurugeon Castle? How does killing Freyedies prevent you from becoming an assasin? I didn't want to kill him but the experience and money was just too tempting.

Thanks all for answering.
I did swim in that lake but no one appeared.
hmmmmmmmm.......