So let me get this straight. Are they both the same? Or Man Eater is suppose to be the counter part of Man Killer? Where Man Eater indicates damage reduction, while Man Killer is Damage boost to Man units?
All of these answers need to learn to not speak up if you don't actually know. Your best 'guess' doesn't earn you participation credit towards a real answer.
none of these answers are good
Here's a direct copy/paste from the WotV Customer Support beginner's guide:
"Killer refers to the ability to deal more damage to a certain type of unit. It functions the same as Man Eater and Elemental Eater abilities. A "Killer" icon will display on the screen during an attack when the effect is activated.
Example ability with a Killer effect: Beast Breaker: Deals Dmg (M) to target & Beast Killer."
man killer = humans. man eater = males
I am no expert and I may well be wrong about this, but here is my understanding. I welcome corrections. Think of eater and killer as suffixes in WOTV. Eater means a percent of the damage heals you so, yes it could be thought of as damage reduction. But if you get over 50, you will actually gain life if "damaged" by that type of attack. Man eater works on any attack from a human, light eater works on light damage, etc. Killer increases damage against that type of unit.