In my opinion (since I unfortunately only have opinions) the cop played the critical part by confronting the criminal face to face. It doesn't really matter who attacks a drug user from behind, as long as it's not a tiny child or a century old, feeble geezer. It doesn't take a black belt to render somebody unable to harm others for the time being if you do it from behind. So, yeah, they both saved everybody there, but the cop was the one who initiated it and thus played the critical role. That's why I first said the cop saved them because he first needed to save Shoutarou so that Shoutarou could have saved anybody. If the cop had remained outdoors he would have been in no danger anymore as he had already called for backup. I'm sure the addict would have taken his sweet time killing everybody indoors in order to chase for the butterflies or whatever he was mumbling about.