I was always puzzled by the true motivation for Vesper’s suicide (‘M’s phoned-in specu-explanation notwithstanding) until I came upon this unused bit of dialog from the end of Casino Royale where Vesper (Eva Green) explains to Bond why she must kill herself:
“Alas… I have been caught in this horrible web of deceit, I have shamed myself & as a traitor I will no doubt be killed by one side or another no matter what I do. I have found a way to spare your life & you have killed off the bad guys, but nonetheless I do not deserve to live or to be loved by you, James Bond. I know at first it seemed that we hated each other, but of course that was all just cinematic foreplay. And I digress… you will be better off without me, and you will get over me eventually… probably by the time the sequel to this movie is finished. So as I quickly and beautifully drown in this disgusting Venice canal water just be assured that I shall head into the afterlife with many fond memories of those things that you do with your little finger. Glub. Glub.”
It even looks like she’s kissing his talented little finger goodbye.