The Emperor is a perpetual. Perpetuals come back to life eventually. Because he's not fully dead, he cannot return. Therefore, if Abbadon kills him, he will return and #rekt shit.
So, we must fully kill him.
I keep saying it, 40k is going to have it's Time of Ending (The End Times) and all the Primarchs are going to come back to life and it'll be Time of Ending until they decide that "points are deadly to model sales" and do, not a setting reboot ala AoS, but a rule reboot.
It all makes perfect sense. Look at the facts. All Primarchs are perpetuals, many 'dead' Primarchs, ala Vulkan and Girlyman have dues ex machina stuff already applied to them, Girlyman is 'healing' and Vulkan will return when all his shit is collected.
The only two troubling ones are Sanguinius and Kurze. Sanguinius is an angel and his legion is Space Vampires, so just have him come back to life, reborn. Also, remember, Dante has the propechy of "A man in Golden Armour will stand before the Emperor against the hordes of Chaos". Dante thinks that's him. It isn't. It's Sanguinius. After the Shield of Baal, Dante is emo, but Sanguinour appears to him and they talk. The only thing that's heard is "Do not lose hope".
Curze will probably come back via Chaos shinnanigans and then during the End Times, he turns on Chaos and goes "While both sides are hypocritical douches, you're the worst, Chaos" and does a Settra (Settra got raised up from death in End Times, then turned on Chaos at the end).
Everybody else is either crying in his room, in the Eye of Terror or doing some other shit, or is a Daemon Prince.