The rules state to adjust the life points on the Monster Life Tracker at the end of each round. Is there a reason why this is not done with each attack? My guess is that it's because of creatures like the Clockwork Golem. This means that the players will have to do the math in their heads as each Attack card is revealed. "Now it has 100 Life Points left." ... "Now it has 60 left." ... "Now 30." Etc. After all attacks have been made, the Tracker is adjusted (or not, in special cases like the Golem).
From the designer:
That is all correct. Many creatures can adjust damage that has been done to them, Clockwork Golem, Spite, etc so it is just better to move the tracker at the end of the Round.
And yes, the creature dies as soon as it hits 0 points. At that point though, there is no sense in moving the tracker.