General: Health based reactive spell triggers floating combat text on dead NPC's
Best seen with execute kind of spells, like the Paladin Hammer of Wrath. This spell is only available to use on mobs under 20% HP.
If the floating combat text is turned on for reactive spells (in interface settings) it will show a message on the screen once the mob goes under 20%, as in this screenshot:
Once the mob is dead, it will still trigger the message on the screen if you click away and then click back on the corpse, as per this screenshot:
However, if the mob never drops under 20%HP and you deal enough damage to kill it while it's above 20%HP, the message will not pop up when you select the corpse.
Summary: if the combat text is triggered, it will keep on triggering even when the mob is already dead, but if it never triggers by dropping under 20% (but not dying), it will not be triggered when you select the dead NPC.