To use Minecraft as an example, a player may die from falling from too high, drowning, getting attacked by a monster. If killPlayer() is called serparately for each of those cases, he asserts that it may cause bugs due to differing context or sequencing relative to other parts of the code. If OTOH you just decrement player health in each of those places and then check for health<=0 at only one place, you eliminate that class of bugs.
To use Minecraft as an example, a player may die from falling from too high, drowning, getting attacked by a monster. If killPlayer() is called serparately for each of those cases, he asserts that it may cause bugs due to differing context or sequencing relative to other parts of the code. If OTOH you just decrement player health in each of those places and then check for health<=0 at only one place, you eliminate that class of bugs.