Confused about the ending (*spoiler*)

I thought this was a good movie, very touching, that is, until the end, when...

He kills her!

Ok, so... The protagonist tells a mutilated Gentille that he will take her to Canada and get her the help she needs to recover. This was a satisfying resolution, proving that he truly loved her, and didn't see her merely as a pretty, youthful body, which was a recurrent theme throughout their relationship.

But, then... he kills her!!! I don't get it. She didn't look like she was in her death throes, beyond any hope of recovery, and he was just curtailing her suffering. Did I miss something?

Earlier on, she had asked him to kill her himself, should she be in imminent danger of being raped/killed by the extremists.
This was no longer the case at the end of the movie; the war was over. If he truly loved her, thought she was dead and yet found her again, shouldn't he have clung to any last shred of hope, and found her life, however fragile, precious beyond any other consideration?
She didn't even look like she could not have recovered; she only had some scars on one side of her face, and whatever other wounds she had, she was still able to survive with minimal help from Desiree for some days, so they couldn't have been that bad.

I really feel I missed something here, because his "merciful killing" completely took me by surprise. Help?

Good movie, otherwise.