Sinister has been a betrayal waiting to happen from the very start, so his backstabbing should surprise absolutely no one.
Henry fully breaking bad is probably the one that should sting the most, but I think not even X-Force is fully aware of the extent of his fall from grace at this stage. For the rest of Krakoa he's probably just seen as reckless, cold and callous but not downright villanous. I hope there is a reckoning coming with real consequences and a huge payoff when it eventually comes into the light for everyone to see.
As things stand now, Moira's betrayal should be the one that hurts the most, except any reaction from the people that should be closest to her (Rahne, Kate, Brian, Kurt) has mostly happened off-panel. Even with her relationships reduced down to Charles and Erik (plus Raven and Irene I guess), we've hardly seen much fallout aside from Charley vaguely mentioning that Moira had defected. Such a shame.