Acnologia became a dragon by bathing in too much dragon blood.
Nope, it's all Zeref's fault. I get that Zeref is supposed to be the main villain, but blaming stuff like this on him is way too forced. He seems more neutral than anything else.
I did thing Acnologia was King of Dragons, but him being a former human is a nice twist. The question is, what does Zeref have to do with it? Clearly he was taught magic by a dragon and was a dragon slayer (unless he learned it from a "Demon" or had latent Godlike abilities), but since he's not Acnologia's Slayer, what is her? The former king's slayer? Maybe Zeref was in fact a former dragon, thus giving him magic abilities and then species swapped with Acnologia, letting him be a dragon in exchange for Acnologia's humanity.
It's still very hard to judge these things, because despite all the good points about Fairy Tail, there is very little information on the antagonists.
Also, thanks for trolling us for 2 weeks Mashima. I guess the 300th chapter was going to be a big deal, but nothing huge happened that made it worth it. If anything, this should have been the 300th chaper.