User blog comment:- Darkchylde/Acnologia's Element?/@comment-173.87.12.242-20141211055642/@comment-4829046-20141214062333

Indeed. There are many details we don't know about. What we DO know are his actions towards Fairy Tail.

And what? I'm sure there are plenty of Dragon Slayers that killed Dragons and absorbed some of their power, but we have nothing to say they slaughtered hundreds of Dragons indiscriminately and became insane with power. And said slayers are very likely dead so we don't need to consider them unless we have a flashback arc or time travel arc. The current First Gen DS have the ability to absorb other Dragon elements, but they do it without killing.