The achievement "Monster Hunter" is not obtained by killing the warden.

What was supposed to happen..

Killing the Warden will get you the "Monster Hunter" advancement.

What is happening..

Killing the Warden will not get you the advancement.

Steps to Reproduce

1. Go into a Minecraft World.
2. Type in /summon minecraft:warden
3. Kill it.
4. You can see that the achievement is not obtained.