User blog comment:Real Godisme/Chapter 481/@comment-7418318-20160415214152/@comment-7418318-20160416080515

In reality I'd be worried about the curse too, but this is FT. Mashima will not kill more than 100 members of Fairy Tail, including Natsu, Lucy, Cana and Makarov all in one fell swoop.

Besides the basement is far enough underground that the others are save (juding by what we've seen from Zeref in terms of range of the waves), except for Cana obviously, who's in the same room.

If Cana succeeds, I'm sure Mavis will immediately go to implement her plan away from the guild. With the town being evacuated there's nobody outside of the guild that Mavis could accidentally kill.