Kasei's actions explained

[Kazuo Shinrai]

The mercenary came as summoned, however, he did not expect to be fighting side by side with his old enemies. When they insulted him, that he could live with. When a lowly peasant follow their example, that needed to be dealt with, no matter what the cost.

Kasei would not have attacked any Covian army as at it would be dishonourable to deliberately weaken your own allies before battle.
GRD was there as requested by the baron, therefor, it would be dishonourable to attack them.
The peasant, well, that was a whole different story all together.

Heh, it is annoying though. I would much rather have participated in the battle against Vesper.