The Blue Bitch is some sort of Eldricth Being like Zhan Tiri or whatever the fuck (I'm drunk and I am not checking the fucking spelling) and you cannot blame Cass, all fucked up from all the betrayal and shit, for not being mentally prepared for that assault.
Cass will clearly be redeemed and fuck, unlike Varian who went crazy and TRIED TO FUCKING KILL PEOPLE without anything egging him on./ I mean I love Varian, but unlike Cass, nothing pushed him to kidnap and try to kill the Queen and eventually the Princessa and her friends, too. So Varian is objectively worse.
But neither of them is that bad. i think part of the point of the show is you can't blame desperate people for being pushed to desperate extremes.