A year after the Grand Magic Games, Gray Fullbuster became a Dark Mage and killed Frosch, which caused Rogue to succumb to the "shadows" plaguing him.

Why does Gray kill Natsu?

He is motivated by vengeance, and this desires blinded him about the fact that END is a nakama. This is proved by Erza managing to stop him by making him remember that no matter what, Natsu is still Natsu. Gray has a very ugly history with Zeref's demons.

Does gray become a villain?

Gray Fullbuster has been especially mysterious as now the character has made a villainous turn, joining the black magic cult, Avatar. Earlier in the series, Juvia had explained how she and Gray had been living together in an abandoned village.