All of these will have their own purpose, and some can be stronger than others depending on what youre looking for, so dont take the self-mill term too literally
You have big creatures like Bloodthirster and Chaos Defiler, and support creatures like Chaos Terminator Lord, Bloodcrusher of Khorne, and Herald of Slaanesh that make your demon creatures into huge threats
Your party consists of up to one cleric, rogue, warrior, and wizard
For a full comparison, read the Play Boosters vs Collector Boosters MTG booster guide
Ramp & Mana Support: Sol Ring, Arcane Signet, and Talisman of Dominance ensure you can hard cast your huge creatures when needed
Beetleback Chief goes more in a Goblin kindred strategy, which is fine, but not synergistic with the rest of the deck