I remember when Dieties and Demigods with the Cthulhu and Melnibone and other copyright violation mythologies were in it. Players and DM’s alike got the idea that if something has stats, it can be killed. I see the stats as a way to indicate the “avatar” of a divinity’s capabilities are. The “avatar” might be “killed”, but the divinity can’t be. For example, a god of magic could cast an illusion of his death. If the players fail their saves and believe it, how would they ever know? If they did figure it out, how would they ever get another chance?
One should be very careful playing at battling/confronting deities. They have countless minions of minute to great power to throw at you. How many saving throws will your +5 Holy Avenger make when it sees the test of battle with a host of evil? If it shatters, can it be remade? If not, is there another one somewhere in the world? Unless you are in a Monty Haul campaign, you won’t have +5 weapons and armor and lots of artifacts to rage against the gods.
I don’t like the idea that a deity can be killed. Players would be messing with the fabric of the universe. Even though we all had grand ideas of getting tough enough to go kill Tiamat, etc. it would be very difficult to do so and would have some sort of repercussions in the fabric of space and time, or tip the scales drastically in good vs. evil or law vs. chaos. If it were possible to kill a deity, make it very difficult, not just to kill the deity, but to get past all their minions. How many huge ancient red dragons can you beat by yourself? Of how many balrogs can you fight by yourself? If it is possible to kill an evil deity, it should be possible to kill a good one. What about an NPC party of powerful evil characters going to kill the head of the pantheon? What’s to stop them from doing that?
Can a wish bind a deity? In Greek mythology, the titans were displaced, but were not truly dead and defeated, there is the far off future when they might rise again. If they are “killed”, how long do they stay dead? Just as a PC can be raised, why not a deity. If they can be killed, then they can be raised.
If you are going to have your deities playing chess with players and other creatures as pawns, a la ancient Greek mythology, then perhaps you want to have a god-killer option, but be sure it makes sense and is consistent. Make it very hard for your players to learn of such a thing and put them through great trials were the dice that come up death to a character means a character dies and may not be able to be raised. Then go against the god(s).