Thanks for playing! Regarding the medkit, clicking on it already sets it as "current health item", which you can see at the bottom of the inventory. Once the current health item is set to medkit, you can go back to the game and press B to use it. I guess it isn't that clear, I'll try to fix that on the next update.
When it comes to the monster, it's the first time I've heard that happening. Did the monster play any sound before standing still? It could be that the monster died, but the death animation didn't play
I hope you enjoyed the game regardless ^^