I'm sorry, what?Nothing is necessary, but the OP seems to be trying to argue that evil gods are redundant, because they can't see a difference between slightly evil and very evil.
And this is where real world religion comes in, by not really allowing for different degrees of evil.
Erythnul - God of Slaughter and butchery who wants you to massacre everything
Nerull - God of Death who literally wants to snuff out all life
Incabulos and Talona are both dieties of disease, poison and death, who want all the world to suffer and die.
Then we have The Beast of Butchery Yeenoghu, who wants to kill everything, Orcus the Prince of the Undead who wants to kill everything and rule it, and Asmodeus.
Which of these is the "slightly evil" side of the spectrum? These all seem very evil to me.