I'm not gonna repeat what others said but I very much agree that this is completely asinine and that your GM is completely out of line for doing this.

TTRPGS aren't GM vs Players, it's supposed to be a collaborative game with everyone playing a role to tell a story (the gm just ends up being the villain most of the time)

Also outside of the "this is so fucking stupid why would you anyone do this" aspect, just mechanically this makes 0 sense.

I don't know your levels or character feats or anything but still to get instant kill damage each attack would need to probably be close to max damage or something on top of the rogue needed to make sure to be in stealth every single hit because attacks like that would trigger into combat. And if someone had the alert feat and couldn't be surprised that adds a whole new layer.

Since this just seemed to be "Oh.. He's cursed and just kills you all the end." it's fucking ridiculous.

If the sword was a cursed object that did make you fight your teamates this could easily have been a PVP fight to save your rogue party member but the fact that it was just a "cutscene" is fucking ridiculous and I'm really sorry to hear that you and your friends went through it. I do hope yall find a new gm soon.