Jack steeled himself as the Dalek approached. This was it, the end for him, no way out, and… he was surprisingly content with that. It was a better ending than he could previously have hoped for.
He didn’t want to die, he was young, with everything ahead of him, there was so much he hadn’t had the chance to do, but the same could be said of his fellow defenders, and why should their lives be less important than his own?
If his death bought the Doctor time, then it was worth it. He was helping to save the universe.