Kain is willing to give a lot of others chances... dragons especially... but darkspawn are on their own level. Different. As a Warden, they're kind of enemy number one.

"That may well be, but my intention is not to wreak havoc and devastation across the world, I only intend to survive. When a bear attacks, he only means to survive as any animal does. But darkspawn are foul things, with no redeeming value whatsoever."