I hate to say this because my heart says let him live, but yes, Kodlak needs to die. Gunnar becoming harbinger is kind of important. Also, the loss of Kodlak but seeing him again in Sovingarde was always so important to me in a playthrough where I'd joined the companions, and when I'd married Vilkas I always imagined coming home to him with the reassurance "he's really cured, he made it there, you'll make it too, I promise".