User blog comment:Xilinoc/Ch. 572 (inb4 "Kenpachi doesn't have Shikai")/@comment-24113610-20140312144930/@comment-8669536-20140312180429

I'm not defending him, but the Sternritter's mission is to eradicate all Shinigami as quickly as possible. The Quincy have already been shown to have no "honour" in this way, their mission is to kill all Shinigami and they intend to do so by any means necessary. It is much faster and much less dangerous to begin by eliminating the potentially most dangerous threat, which at the moment is unable to resist(the captains who would be dangerous indeed were they to be healed), and then go on to the lieutenants, the lesser threat.

So it's not really cowardice, but rather he is following his orders and going through with his mission of extermenating all Shinigami. From the Quincy perspective, this is not supposed to be a war, they want it to be a slaughter. Though if he were smart he would have killed Yachiru and Isane while they were busy fighting Guenael.