Recommend me a full empire build based on this information

[Alxndr]

I want to create an empire whose sole purpose is death, annihilation, destruction and slavery / brutality.

I want every single empire to hate me and refuse to even speak to me. I want to conquer the galaxy exterminating everything in my path without the need for a casus belli. I want war just because its in my blood.

Recommend me traits, civics, race etc for this kind of role play but stuff that's also strong. I've just finished a peaceful democracy based run, now I want to do the complete opposite.

[Chap]

race doesn't matter unless you want species specific traits, if you want to annihilate everything then existential iteroparity is a good trait.

everyone hates you if you are a genocidal empire (fanatical purifiers, devouring swarm, determined exterminators only hate and are hated by biological empires, driven assimilators get a -400 stat to other empire attitude towards you but can engage in normal diplomacy)

everyone hates if you genocide other species as a non genocidal empire so when the galactic community forms join it, wait for other empires to contact you and then genocide everyone because they won't hate you if you genocided before they contacted you

barbaric despoilers have the plunder casus beli and have the raiding bombardment unlocked by default so you can abduct pops from other worlds and send them to yours to be slaves/genocided.

fear of the dark origin lets you play as a normal empire for the first 100 years of the game but lets you turn into a fanatic purifier for free (even the fanatic purifier civic you get doesn't cost a civic slot)

becoming the crisis progressively makes everyone hate you and at crisis level 5 everyone declares war against you with a total war casus beli

pompous purists civic stops other empires from being talking to you

[HappySack]

Sounds like you just want to play a genocidal empire type but they're not allowed to have slaves so Barbaric Despoilers is the closest thing as it's more of a space barbarian kind of playstyle but the second civic will vary.

Edit: I don't know if this works anymore but Fanatic Purifiers can switch to the Neutering purge type and put all their xenos on the same world to make a "torture world" which gives free unity for every xeno on it and since the purge type is very slow it will make the pops last for a long time

[Cerodil]

Here's my current run's build, I don't think it hits all your points but it's close. and I've added my thoughts of why I choose some things.

Origin: Under one rule

Ethics: Fanatic Authoritarian(going to get lots of slaves)
Militarist(optional took it for a civic, though it does have a stupidly good paragon general)

Civics: Philosopher king(very strong with under one rule, build your fleets up and when you go imperial you will never run out influence. The councilor position is super strong and helps offset perfected genes)
Distinguished admiralty(only real reason I'm militarist but the councilor position is good if you can't get a merchant or curator)

Traits: Rapid breeders(need lots of rulers to own all those slaves I'm going to have)
Intelligent
Traditional
Unruly(Easily one of the best negative traits, you can get rid of it very early)
Solitary(Oh no not housing...)

Planet: I picked arid for the increased chance of energy districts.

Leader traits: High King(helps counter warmonger and makes rulers op. If you're lucky you can get this to level 3 before un-pausing because of Distinguished admiralty)
Inspired Commander(goes well with Distinguished admiralty, you wont need the social military techs for a while)
Warmonger(meh)

Make sure you set the default slave type to "indentured slavery" otherwise they wont be able to work as specialist. Make sure your species rights are set to "Stratified economy" and only your rulers happiness will matter. Then just get as much governing ethics as you can so everyone joins the authoritarian faction(promote it as well) and you'll be swimming in unity with high stability as long as the slaves have jobs they wont(can't) complain much.

[ChaffyExpert]

Fanatical purifiers sound right up your ally, or Barbaric despoilers, get free CB on everyone and can abduct pops.