This is just a small theory, but Acnologia uses magic, as all the other dragons, so that would mean he would be powerless with Face activated, so he wants to destroy the Face and punish Tartaros for trying to take out his power?
I think it's just there to wreck shop, IMO face won't affect it. Probably realizes all of the things taking place and just wants to be a part of the fight. This arc is taking a huge turn though for sure, can't wait to see more!
Thia got me so excited i was not expecting acnologia to show up with tartaros, and even mard greer still fighting. I think he is there to destroy face as the user said above or i think he will fight end and it will be a colossoal showdown because igneel is on his way as well as grandeeney and metallicana....... This is crazy, but can somoene tell me what happened to the first master after she had that talk with zeref....
I don't see Acnologia destroying face unless he has a self destruction spell because that is the only way to deactivate Face now. Destroying the physical body of face won't stop face at all as shown when Wendy destroyed the actual bidy of face yet it still as counting down. Although I seriously think Hiro will forget about that since he forgot about Juvia being poisoned.
If Acnologia uses Magic things aren't looking good for him. He will probably turn back into a human and we will finally see how he looks. The best acnologia can do right now is wreck havoc for the time being until face goes off.