As I said upthred, it wasn't abou protectng M, it was aout protecting everyone else.Number of innocent civilians killed by Slva after Bond kidnaps M? Zero.
Though that would also have been the result if they'd stashed M elsewhere and left the "trail of breadcrumbs" to Skyfall where Bond was waiting.
However, I thought there was a definite subtext - and referenced in dialogue - that M goes with him because it's one last field mission with a chance of killing the guy who'd just fucked up her career, and a chance of getting killed- which would be preferable to her than the enforced retirement she faced otherwise. She went willingly, with her eyes open - she was taking the Long Walk, as it were.
Pretty much it.. M wanted to go out with guns blazing (even though she's a bad shooter
) and prevent civilian casualties. Her man is dead (must've happened sometime in between the movies because i do recall a man in her house in Casino Royale when Bond calls her private number) and she's not the type to sit home alone and watch daytime soaps.
Her death scene was pretty sad.. even more so when Bond has to fight back the tears and prevent himself from collapsing.
I liked the contrast between him and Silva too when it comes to M. She would have "sold" out both of them if she didn't have a choice or the stakes were too high like they were in Skyfall when Bond fought that guy to retrieve the list. Bond understood that and he understood the rules of the game.. he knew that there would come a time maybe when someone would give the order to abandon him for the sake of country and the mission.. Silva couldn't stomach that and took it personal.