He should be p@ssed off ,I would be too. I don't want someones cattle in my yard, I don't let mine roam wherever they feel like so I expect others to keep good fences .
However, shytt happens .
It is your responsibility now to pay for and fix any damage that your cattle did in a very timely manner, no matter how little you think of his pond, it obviously means a great deal to him. He may not be able to shoot them ,however they could come down with a mysterious illness and you will have no proof of anything ,just a handful of dead livestock. Make sure your fences are bomb proof ,so there is not a next time .