Evil is evil. Who decides what evil is necessary and how?

If you start deciding to allow “necessary evil”, how do you draw the line?

Say you draw a line. That line is further into evil and is then perceived as the “new” line between good and evil.

Then someone else commits a “necessary evil” which is further along evil and the first “necessary evil” and so on and so forth.

It is a slippery slope. Let us try to be as good we can be rather than dabbling with concepts like “necessary evil”.