Arthur's current mentality was that of a goblin's chief. It was a side-effect of the skill Goblin's Bond, as it went two ways. Of course, he could have decided to leave the goblins to their demise but it would have left a bitter taste in his mouth.
Another reason was that he realized the goblins village would be attacked if it was found, and Kira was still there. He had to get to the village and hide it before they were found.
As he felt the dagger tear through the back of the man's back, Arthur retreated in a hurry when he felt the other person coming to attack him. He dodged in the nick of time and used his latest offensive rune, Fire.
This rune was like a gate that would turn his mana into a fire and manifest it into the world. He could command this fire, that was originally his mana, as he wished. He could condense it, spread it, increase it, or decrease it.
