Here is the full list of all 29 KILLBUG achievements.
Kill a god
Reflect 666 projectiles
Kill a Larva Hive with only its projectiles
Kill a Tapeworm
Kill a Tapeworm quickly
Destroy 6 spider eggs quickly
Regain 666 health
Recover 500% of your health in one run
Anger 42 centipedes
Stay in the air for a long time
Climb back up after nearly falling into the mist
Kill a Larva Hive
Die 13 times
Kill a bug with the blood shot
Reload your gun 69 times
Die to a ladybug
Perform a ground smash
Kill 10 bugs in one melee swing
Kill 10 bugs in one ground smash
Recover a lot of health from one centipede
Kill 2 bugs with 1 demon shot
Recover health after almost dying
Kill a bug with the demon shot
Reload your gun when fully empty
Kill a bug with a reflected projectile
Complete the tutorial
Perform a knife jump
Fall to your death
Kill a bug with an air dash