The only characters you need to worry about killing off are the camp counsellors. Killing everyone on a playthrough should be fairly straightforward but here are some tips as some death scenarios cancel others making it harder. (Note: Dylan must remain uninfected).
Note: You may want to pair this achievement with "The Final Girl" and "Last Man Standing" to save time using Chapter select. For additional notes see "The Final Girl" and "Last Man Standing".Recommended killing methods of the characters: Jacob - In Chapter 1, steal the rotor arm, and later in Chapter 3, at the lake dive in after it. When getting tangled, repeatedly "Pull" instead of untangling and Jacob will drown.
Emma - In Chapter 4, open the trap door before searching the bag on the island and Emma will die.
Abi - In Chapter 6, do not shoot Nick when he is transforming. He will kill Abi (also earns "Lovers' Quarrel").
Laura - In Chapter 7, do not drug Travis and be difficult to him while captive. Shoot Travis once when escaping. In Chapter 9, Travis will kill Laura as revenge.
Nick - In Chapter 8, Laura will shoot imprisoned werewolf Nick if Ryan does not interrupt.
Dylan - Make sure he comes to the scrapyard and when operating the crane, warn Kaitlyn and distract the werewolf. Fail the QTE with the torch.
Ryan - Refuse the bite from Laura and shoot werewolf Chris later in Chapter 9 to cure Max. Ryan will bleed out after killing Chris.
Max - At the start of Chapter 10, choose to swim to shore and Caleb will kill Max.
Kaitlyn - Fail QTEs when fighting against werewolf Caleb.
Credit to ThePugHybrid for their
guide.