It isn't just that he murdered Sarah Everard, it is that he used his police warrant card to arrest her and drag her into his custody before putting her through hell and killing her. So she was duped into getting into the car with him and public confidence in the police, particularly the Metropolitan Police, was seriously undermined.
If I recall correctly, the next morning he also took his children to play in the woods close to where he had buried her bodly.
I think the judge made the right call. Evil monsters such as him should never be released. As some have also suggested, there could well be other victims out there somewhere. Unlikely that he has only struck once, and if released then he would almost certainly strike again. He would be a huge danger to the public, and probably especially to women.
Fixed term is exactly as the term suggests, a fixed number of years to be served before becoming eligible to apply for parole. Whole of life tariff is when the judge feels that the defendant is too dangerous to ever be released.
I agree with whole of life tariffs in cases like Wayne Couzens, Ian Huntly, Peter Sutcliffe (Yorkshire Ripper).