His finger wasn't anywhere near the hammer, yet somehow he hit it.
Rough edges on the wood are what files and sandpaper are for.
And sucking on the finger is typically to get any foreign contaminants out of the wound. Usually one would be doing that personally though, rather than having someone else do it... BUT his cut is probably in better shape after than before, so treating Kotone like she is some sort of komodo dragon is just rude.