Is it mean wanting to put my dog down after the way it killed a cat?

Ever since Feb, it's been having behavioral issues, acting too territorial, barks and growls at everyone that walks near my house and I have to end up putting a muzzle on him. I had to once get him back inside because he was trying to step outside. Plus he has tried dragging me during some walks and it's now taking the 3rd command to get him to stop pulling. He's a Presa Canario.

Then last week, I saw the way it killed a cat. Luckily it didn't belong to anyone. It was like I was watching some scene from a horror movie. He didn't obey my ''stop'' commands. I said it loud and clear. Nothing. I feel as if I can't control him anymore. Then I'm going to likely be responsible if something bad happens. So is it mean wanting to put him down after what he just did with the cat? I saw something in him that scared me. Either way, I feel as if I've failed him. I don't have no idea what went wrong. I've tried my best to provide care and love; never abused him.

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I think my only mistake was being a first time owner of that breed.
Is it mean wanting to put my dog down after the way it killed a cat?
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