Death that don't make sense and why:
Psylocke: because after drinking the crimson dawn she became a creature of mystical energy, maybe the skull creature was welcoming her as some sort of asian valkyrie.
Colossus: The Legacy Virus shoul never have taken him out, as the virus only attacks beta or lower mutants and humans, if you look at all the people who were infected you will see that this is where the plot was going, and the fact that the virus was engineered by Stryfe who was weeened on apocolyptic ideals further strengthens the case. The virus would never have taken to him, someone like Beak or Skin would have been likely test subjects, not Colossus as evident in AOA he was a high ranking alpha.
Blink: A mutant can't die by their own powers unless they are affected by the legacy virus.
Synch: Everrett's powers or character were never really understood by any of the writers as neither one was ever consistant, but the original character was able to retain elements of powers he had come into contact with for a few hours afterward, and could also get into syncronicity with Banshee when he was miles away, so him not being able to synch with Monet makes no sense at all.
I know these three aren't X-men but they could have potentially been X-force members, they are the three most likely hellions to have survived, roullette being least of the three.
Tarot
Roulette
and
Catseye: Due to the nature of their powers I don't think any of these three could be killed, this has been proven by the return of Tarot.