Well, yeah? Emo-edgelord protagonist immediately concludes that being hit by a car with all the repercussions that'll have it might be better if she were dead instantly. Don't see how else you can read that scene? it's literally spelled out that way.
Well, at this point of the story we are already sitting in the MC's train of depressive thoughts, and we are aware that he is suffering the loss, life is pain, people around are annoying, and he is trapped in this shitty reality. And this car accident just gives him an instant confirmation of his thoughts, and I would say he is projecting to the external world, assuming the worst - that the girl got a serious injuries and will suffer from the severe pain for the lifetime, and the only way to avoid it for her is not to regain consciousness at all. And at that moment he says out the conclusion: maybe it would be best to put her down under this circumstances. Of course that's not his intention to kill her, it's just an echo of his depressive dialog being vocalized. Isn't it is obvious?
And then he acts controversionally to his words, advising to stunned Marla to call the ambulance, and putting Vic into the recovery position (I don't know much about this thing, but from my opinion it is a very bad idea for an unprofessional to move a person with the possible spinal injury)