In John Gardner's The Liquidator, the old Chief explains the problem to Mostyn: too many leaks, too many double agents and defectors, too many embarrassing public trials exposing the Ministry's inefficiency - all because they can never bring evidence against their suspects.
"If only we had a way of acting without evidence," murmured Mostyn, Almost sacrilege, he thought, to suggest something which might imperil the sacred red tape.
"Well that's it, old son. That's just it. I've formulated a plan: drastic maybe...distasteful too, I shouldn't wonder, but I can see no alternative...as soon as we get the slightest whiff of trouble, the smallest sniff of a serious security risk, then - snap!" The Chief was nothing if not dramatic. "We liquidate 'em."
"We do what?"
"Liquidate 'em. Dispose of 'em, shoot 'em, give 'em the wooden overcoat, the deep six, the perpetual freeze, the big sleep, the chop. You follow the drift?"
"What are the chiefs of staff going to say?"
"Mostyn, have you not got it yet? We are not going to tell the bleeding chiefs of staff, nor the blasted PM, nor the adulterous Home Office, nor the sodding Foreign Office. And, particularly, we are not going to tell the fornicating Special Branch." (pp 25-26)
And so "L" is born.
I know that this is not canon (it is, however, my favorite exchange in the book), but the sentiment would be pretty much the same: 007 answers to M - the same M who, as Dustin points out, left Tiago Rodrigues to the Chinese when he got too stroppy. The only difference between the 00 section and SMERSH is the latter operated with the full knowledge and support of the Soviet presidium. In CR, a bullet was more expedient and less expensive than a trial - and less embarrassing, provided Bond does it cleanly and without leaving evidence.
And Bond is our hero.