After reading Jak's comments I was totally convinced, so I decided to delete everything proved to be false
The reason why Tartarus kills all Senate members, is probably because they're afraid of Etherion. All of the then-and-now Council members know how to cast Etherion, so they are killed. The same goes with Jellal and Ultear. Jellal has the power of a Wizard Saint, even if he can't handle the murderer he could still flee by using meteor. But Ultear has nothing now - so she is likely to be killed. As a result, Gray/Jellal/Meredy and everyone of FT would activate Nakama
Bullshit Magic, which would be the magic most effective on demons.
Sorry for my bad English. :/