At the end he says "they're not gonna stop", so basically, he had to kill himself and them to save the girl and her son and the cure. Leaving the grenade then getting into the tunnel wouldn't have worked, there would've been time for the monsters to run away, and he had to wait for the glass to break so that the grenade actually got where it was supposed to go.
Thought it was a good film, not one I'd watch over and over, but it was good, even though I found it depressing.
/ 
