He ruined it for him...
I mean that thing, whatever it is was in there for A LONG time, long enough to make this place his ''home''. He propably killed a lot of people all this years in there, but his behavior in the movie was bound to get him killed even if she didn't.
It propably killed or trapped people 1 at a time. But now he went on a killing spree which would attract all the attention of the police etc etc to search the subway and find EVERYTHING and eventually him too.
So either way he would die..