Using the /summon command with mobs that normally possess weapons spawn without them if you use an NBT Tag. 

Steps to recreate:

Use the command /summon vindicator ~ ~ ~ {CustomName:"\"Johnny\""}

The Vindicator spawned will attack everything, and have the name Johnny, but never draw an axe. If mobs are attacked by him, it will do less damage due to the lack of a weapon. This also affects skeletons, as they will attack with their hands, and pillagers get affected more adversely for they use crossbows and cannot attack at all.

This also works with /summon skeleton ~ ~ ~ {Invulnerable:1b}

The skeleton spawned will be invulnerable but will not possess a bow.