Glad the cat is on the mend.
If a dog comes into my garden and attacks my cat, may I kill the dog?
As has been posted ^^ as long as it was reasonable/last resort, I would have thought yes.
On that note, a guy at work was in the position where his dog was attacked (in a public place) by another dog off the lead, which wouldn’t let go or stop the attack despite being kicked etc. After warning the other owner, he took out his *knife and cut the attacking dogs throat. Police were involved and no action taken as it was deemed justified.
*Dog was out for pre-work walk and knife carried for work purposes.