Online Games Where You Play as a Zombie

Casual / .io

• ZombsRoyale.io — BR as zombie.
• Zombie Royale (zombieroyale.io) — Zombie abilities.
• Braains.io — Tag: humans vs. zombies.

Co-op / Shooter

• Left 4 Dead 2 — Versus mode: play Special Infected (Hunter, Smoker, Boomer, Tank).
• Back 4 Blood — Swarm PvP: play Ridden.
• Dead by Daylight — Killers include zombie-like (Nemesis, Blight).

Survival / Sandbox

• Project Zomboid — Mod "Play as Zombie" / sandbox settings.
• Minecraft — Zombie servers, morph mods.
• Roblox — Zombie Attack, Zombie Uprising, custom modes.

Mobile

• Zombie Tsunami — Horde runner.
• Into the Dead 2 — Runner/shooter (human, but zombie-centric).

Pick by platform and preferred pace.