Ever since Judi Dench gave Pierce Brosnan's Bond a good metaphorical dressing down in 1995's "GoldenEye" ("I think you're a sexist, misogynist dinosaur: a relic of the Cold War"), her character of M has been a firm favorite with audiences and the spy alike.
The entirety of "Skyfall" builds up to M leaving the series, and the ending delivers in style. In the beginning, M makes a judgement call that results in Bond getting shot. She is "gently encouraged" to consider retirement, something she is absolutely resistant to. In the middle, we find out more about her past before heading up MI6, which tarnishes our perspective of her as a leader. While heading the agency's operations in Hong Kong, M traded MI6 agent Raoul Silva to the Chinese government, in exchange for six other prisoners and a guarantee of a smooth transfer of power from Britain to China. The idea that M isn't perfect suddenly makes her leaving MI6 seem more tolerable.
In the end of "Skyfall," despite Bond's best efforts, M dies in his arms. Although it's a sad moment, it's become clear that one of the themes of the movie is M's uncertain future at MI6. The character doesn't seem like the retiring type, and there's more than a whiff of ageism in the suggestion that she should give up this position of power and go potter around in a garden. Having her die in a firefight conveniently opens up the franchise for fresh blood, while also giving a beloved character an ending that fits her commitment to her job and her indomitable will.
