I think you should forgive him because he has just made a mistake.  This is not a killing.  Because If he loves you, then, of course, he also loves your dog. This is called triangle love.

I can say from my own experience that I love Allah and I love all the creations of Greatest Allah. Because Allah says in Holly Quran “If you loves me then love my creations”

So I think you should forgive him.
Please think deeply, If you punished your boyfriend, then you will lose your boyfriend forever who loves you.
After punishment, Will you get back your dog?
No, you will not get back your dog forever, So try to forget it and buy a new dog.

But if you forgive your boyfriend then your boyfriend will stay with you with more love and satisfaction.
You could buy a new dog and train as you like. You will get a lot of tutorials for your dog training here.
I think it would be better for you and you will be happy.

Thanks.