No - killing them in self defence (or the defence of others) when no other option reasonably presents itself is fine (fine as in 'not evil').
A Wood Elf who launches a crusade into the Underdark on a quest to raze Drow cities, slaughter and enslave every Drow man, woman and child until there are none left, is evil. He's just as evil as the Drow themselves.
Understood.
So, we let them build up strength, and because they are evil (objectively so) they will then attack us to rape, enslave, and pillage. So we kill a bunch of them until they are too weak to attack us again (therefore in self defense) and then wait for them to get strong.
Then they will attack us, killing, enslaving, and pillaging (again, objectively evil, they will never do anything else) so we will cull their population again.
And, like non-evil people, we will just let this go for all eternity.
Sound about right?
Depends on what they are doing. If you happen onto a remote tribe of orcs and just start killing them, it could be an evil act. If there is an orc tribe that is raiding local villages and killing villagers, killing them is not going to be an evil act.
But that remote tribe is evil right? Orcs are evil. So as soon as they see you, they will attack you to rape, enslave or kill you.
So, attacking them first doesn't matter, they will attack you, you will fight back, and either you are weaker and therefore lose, or you kill them all.
This is a fact unless the DM changes it. There's nothing to "accept."
Why would that be a guarantee? The DM is free to depart from the default for individuals. If he wants to change the default, though, that's a home brew.
See, because here is the thing. RAW, all orcs are evil. All goblins are evil. Ect. Creatures that have a choice have "any alignment" listed.
After all, evil is objective. If you see an orc, the DM will look at the statblock in the Monster Manual. That statblock will say Evil.
Sure, they can homebrew it like you said, but every orc is evil. Objectively. That is what it means to have objective good and evil in DnD, and to use the statblocks as RAW for alignment.
Every Orc is a, what does the MM say, "savage raider and pillager" who "gather in tribes to exert their dominance and satisfy their bloodlust by plundering villages, devouring or driving off roaming herds, and slaying any humanoids that stand against them"
You are free to change that for your games if you like, but that is RAW, can't argue against it.