tbh why didnt they let the Order in on Dumbledore's plan to have Snape kill him? Like all of them in the Second War were very loyal to each other, it would have saved a lot of heartbreak and stress if they were actually let in on the damn plans. I mean it get it plot wise for the "plot twist" but its annoying that so much of Dumbledore's plans seemed perfectly fine to let Snape suffer worse and worse.
most simply it’s as you say - a plot device to keep readers guessing about snape until the last possible second.
however, in world it does make sense. we know that dumbledore is secretive and manipulative and that he often keeps information hidden even (almost especially) when it should be shared. even with characters he (supposedly) loves, he has troubles with communication - harry would have benefitted from knowing what the fuck was happening to him in fifth year, but dumbledore kept mum until he literally was forced to tell harry.
i don’t think snape rates as high to dumbledore as harry does. i think dumbledore has some affection for snape, but it’s the kind of affection someone has for their favored weapon or a particularly loyal pet. dumbledore can be kind and thoughtful to snape, but snape will always be a spy first and a person second to dumbledore.
and for snape’s safety and, more importantly, for the safety of his position as a spy, everyone needs to think snape is evil. everyone needs to really believe that snape was never with the light. i don’t think dumbledore thought about how that would affect snape - not because dumbledore is some sort of monster or because he doesn’t care about snape in his own way, but because for dumbledore (and honestly for snape) snape’s spying is just more important than anything else. keeping that position is always the first goal - snape’s safety and happiness always come second.
and that’s fucked up, but war is fucked up and i think dumbledore more than anyone knows that. (remember, he’s already been through this before - not just with voldemort, but with grindlewald.) and i agree that the second war order was loyal, i don’t think dumbledore ever forgot that nasty shock about peter pettigrew either. and i’m not sure he was entirely wrong about that - there are clearly some leaks in the order.
what i don’t get is why dumbledore didn’t at least tell mcgonagall (who is clearly dumbledore’s chosen successor as leader of the order) so that snape’s information could continue to be put to use. like i think of all people dumbledore would trust her and if at least one person knew then snape could continue to feed information to the order. my best guess is that dumbledore feared any connection to the order would be exposed and it was better not to take that chance.
and, to be honest, i don’t think it’s entirely cold-blooded on dumbledore’s part either. the moment snape is exposed, his life is forfeit. keeping his spying position as safe as possible is also a way of keeping snape as safe as possible. in an entirely selfish way, of course, but still.