It seems to me that so much fuss is being made about a muttered remark, the likes of which I feel quite sure most people - and certainly MPs - have muttered (or even spoken aloud) when they are confronted by boorish behaviour.
However, explicitly violent and, in my view, dangerously provocative words have quite recently been used by Conservative MPs against May, as were widely quoted in the press in October:
"The Sunday Times quoted one unnamed Tory MP as saying: “The moment is coming when the knife gets heated, stuck in her front and twisted. She’ll be dead soon.”
"Another said May was now entering “the killing zone”, and a third remarked: “Assassination is in the air.” In the Mail on Sunday, a source was quoted as saying that May should “bring her own noose” to a meeting of backbench Tories.
"A No 10 spokesman said that there would be “no investigation” by the whips into which MPs had briefed Sunday newspapers.
"One Tory MP and public critic of May, Mark Francois, said the language was “unacceptable” but added that that he would not tell the Conservative chief whip, how to do his job.
"He said the language was born of frustration ...." [Oh, that's OK then].
So, it seems the Conservative Party was not prepared to investigate what I believe many people would consider to be the unacceptable use of violent language directed towards their leader. But they are quite happy to express outrage at Corbyn for muttering "stupid woman/people" about their leader/or party.