User blog comment:Omega natsu2/Fairy Tail Zero: Game of Truth, Review./@comment-5894890-20140820194116/@comment-108.28.233.191-20140822044238

Fairy Tail's heroes have never been very big on saving their opponents. For example, in the Tower of Paradise arc, they never gave a second thought to the fate of Trinity Raven, and the random mooks they beat up, even though the tower was ultimately vaporized. Oracion Seis arc, they just kind of left Brain, Midnight, and Cobra on the collapsing Nirvana even as they worried about getting clear themselves. Tenrou Island arc saw them unconcerned about the deaths of Zancrow, Zoldeo, and Azuma. Sun Village arc saw them unconcerned about Doriate's fate. And of course the Tartarus arc has seen them finishing off various members of the 9 demon gates.

So Gray not saving the time bomb guy from his karmic end is fairly par for the course.