Although Valthorn didn't like Kaizen, and neither did Kaizen like the leader of that fleet of dark elves, they were forced to put up with each other.
Kaizen needed Valthorn alive to obtain crucial information about Niflheim and the lord who ruled over the region known as the Frozen Plains. For this reason, the player supported Valthorn, and only for this reason.
Valthorn was no different. He didn't trust Kaizen at all, in fact, he even felt a little disgusted just looking into Kaizen's eyes, but he could feel with every fiber of his body how powerful Kaizen was, so he accepted the deal proposed by the human.
When Valthorn and Kaizen shook hands, it was Talfor who felt most relieved. He felt he could trust Kaizen, and perhaps with another enemy from Valthorn's perspective, the dwarves could finally have some peace.