Question: how to make butter slime not hard?
Answer: Add a bit of water. Sounds too simple, right? ... 
Mix in some hand sanitizer or antibacterial gel. Another way to soften slime is to add a drop or two of hand sanitizer. ... 
Moisten it with lotion. Lotion keeps your skin nice and soft, and it can do the same for your slime. ... 
Add more glue.