...it is unlikely that a hero dies since a dead hero is better than a barely living hero.
Every living hero receives an extra crystal for every dead hero which gives them more flexibility (might not sound much but 1 crystal often makes the difference)
A barely living hero with 1, 2 or even 3 crystals isn't a big threat anymore and doesn't help the others as much as these crystals would do if they were on a healthy hero.
So, if the overlord wants to win he is aiming for the 1 - 2 hitpoints on every hero.
Not always true. On the first scenario I once let Shevatas live with only 1 crystal. Later they threw him Zogar Sag's head and he ran
to score a touchdown outside of the board