User blog comment:SoulRise99/Sage Dragon Slayer Magic?!/@comment-1776824-20170222003110/@comment-30462872-20170303162801

My opinion? Tenrou, Acnologia went there to kill the dragons, he merely saw the Dragon Slayers as children, not a threat to him, and the vessels of the dragons. Alvarez War, he's actively targeting them. As to why, I'd say he sensed Natsu gaining the power of the Fire Dragon King and realised he couldn't afford for them to live given Igneel was able to tear off his arm on the brink of death. That power in the hands of Natsu in his mind is probably unacceptable and he's resolved to wipe out all living traces of the dragons in case the others gain similar powers and become a completely legitimate threat to him.

Acnologia doesn't seem like he's stupid. He figured out Irene created Dragon Slayer Magic just by looking at her corpse, and she was a High Enchanter by a single encounter with her magic. Also, he killed God Serena before he finished talking, I don't think he wanted to give the guy a chance to get a single attack off. Judging by how he landed outside Fairy Tail deliberately, he was aware Natsu and Zeref were there and fighting, I reckon he wanted to take advantage of them fighting each other so he could kill them both off easier. Wendy was also immediately outside, so he could go after her easily given her wounded state as soon as he dealt with the Dragneels. From there, I'd guess he'd go after Sting, because he's got two elements now, then Rogue because he's with Sting, then Gajeel, then Laxus, and finally Cobra. I think that'd be the order he'd go after them, because Natsu has the power Igneel once held, Wendy can heal most wounds even if they're potentially fatal, Sting has massively amplified powers, Rogue's second to that, Gajeel just came back from Hell, Laxus is in a somewhat tired state, and Cobra is in all fairness probably the least adept Dragon Slayer.

Just an idea, but I think he's been shown as familiar with E.N.D, so he might know Natsu IS E.N.D and want to take him out before he awakens his demon powers like he did when he fought with Gray.