For my own part, "god"/"deity" connotes something more...fundamental to existence than these other things. A powerful devil is, to be sure, a dangerous supernatural force with a particular agenda, and an evil god is likewise a dangerous supernatural force with a particular agenda. But "godhood" connotes something more deeply-tied to the nature of reality.
This is part of why I prefer the 4e conception of what godhood means, as compared to earlier editions. In a very real sense, 4e gods are living concepts. Something about what hope IS, exists in Bahamut and in Pelor. Something about what storms are exists in Kord. Etc. When you kill Tiamat in the end of Scales of War, you aren't JUST defeating a powerful supernatural force of evil. You're literally making greed, envy, and malice less impactful in the world. That doesn't mean greed will cease to exist, but it does mean that her death should herald an age where the things she embodied are weakened:. Charity, kindness, and benevolence will flower in the wake of her destruction. These could, of course, eventually become problems in their own right (e.g. these good things being warped into extreme and oppressive things), but at least for the time being, vice will be diminished and virtue will thrive.
You wouldn't get that kind of result from killing a "mere" fiend or celestial. Kill a powerful angel and sure, the forces of good have lost a powerful member, but you haven't damaged the cause, you haven't hurt Good-ness itself. Take out a succubus, even the queen of the succubi, and you'll certainly cause a stir and probably weaken Abyssal ambitions due to the resulting infighting, but you won't make Evil-ness less prevalent. Killing a deity-level figure, on the other hand, has serious implications that go beyond the direct personal schemes and servants of the dead god.
I'm using an idea like this in one of my settings.
The gods are living concepts of aspects of the world. When a god has a god-child and that child god comes to take own an aspect, that aspect is born into prominence. Spring doesn't become a thing or doesn't gain impotence until Persephone becomes Queen of the Underworld.
Likewise a god can be spontaneously created by a worldly aspect gaining power. This is the Eldar spawning Slaneesh in 40k by being too indulgent.
So a powerful devil, demon, or fey doesn't become a god until they embody an aspect of the world and take that spark.
So your Evil God of Murder should kill another god. Your Evil God of Undeath should have created an new kind of undead and took an undead creature as their heralds.