Destruction of private property. If the deputy couldn't find a crime with what happened he needs to be fired.
No he doesn't need to be fired.
It may not be as simple as you make it.
Was it destruction of private property? Sure. Was it trespass or a privacy law being broke? Harder to say? So is it criminal or civil? The distinction of unlawful discharge of a firearm can be explained by not knowing if he was in an area where it was legal or not. Presumably it was legal to discharge his gun there (since it mentions no charges).
Likewise, it may be okay to discharge your pellet gun into the neighbors cat/dog and the police might not be able to do anything about it. But there is always civil court to go after as well.
Not all things are cut and dry....