It's kind of both ways. Your impulse is controlled by your amygdala. Fight or flight, where "fight" in this instance represents running into a building to save others.

However, being able to reflect on your first impulse means you will sit back and assess the situation: "Can I get in?", "What are my chances?", "Am I protected enough?", "Will the structure collapse soon?", etc.

So your first instinct may be to run in, but you hesitate because you want to do that critical thinking first.