To answer your questions towards the end, he didn't know Prim specifically would be a victim but he was the mastermind behind the double bombs to lure in medics. Which is, of course, a war crime. And as he said earlier in the movie, he doesn't care about civilian deaths if they serve the cause. The book goes into more detail of course but as Katniss had said "I of all people know that killing is always personal" that was the first time it had been personal to him. (There was the firebombs of 12 but that was mass tragedy, but he hadn't been on the killing end of that--his own violent actions had never had real and tangible meaning to him as individuals and the value of their lives before Prim)