How can I forgive myself or see my dog the same way after what he has done (TW: animal death)

Trigger warning: animal harm/death

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These days I felt the first time ever thinking to myself, why the fuck did I choose to have dogs. How envious I am towards human parents, they usually don't have to experience this shit. Sorry, I just have to get this off my chest. I've lost everything in a few seconds.

How I'm supposed to see our dog the same way again?

8 days ago our dog, rough collie (the "Lassie"), killed our kitten. We took in a deaf kitten with a head tilt. My partner and I are completely devastated, and I can't make peace with what happened or understand how I'm supposed to see our dog the same way again. My partner is coping alright but I'm feeling like I'm living in a bad movie.

We adopted the kitten less than three weeks ago. She was about 10–11 weeks old, deaf, but still incredibly energetic, affectionate and fearless. She was healthy and had been checked by a vet. She was an absolute little lunatic in terms of energy, but also extremely loving and social. Best little thing I've known. She purred just looking at us.

We were still in the process of introducing her to our adult cat, Aurora. The introduction wasn't finished yet — Aurora was still setting boundaries and wasn't completely comfortable with her — so we continued keeping the kitten separated when we couldn't supervise them.

We also have dogs, 2 elderly rescues, and Davion, our 5 years old purebred rough collie, he was my first dog.

What makes this so impossible for us to comprehend is that Davion has an extensive history of living safely with cats since the puppyhood. I took this breed because it was supposedly cat safe. He lived with Aurora the entire time. Over the years he has lived with several other cats we fostered including kittens, and we've fostered kittens around him. He has never EVER shown nothing but love and curiosity, no aggression, predatory fixation or anything else toward cats that made us consider him dangerous. The only thing he didn't enjoy was someone stepping on his fur, but even then he just stood up and signaled to back off. But if he as much as showed teeth... he'd be strictly supervised forever.

Davion and the new kitten had also already been introduced. They had shared a room while we were there and their interactions had been affectionate and completely uneventful. We had no reason to think they couldn't be together.

The kitten absolutely hated being alone and would scream when separated from everyone, so we were experimenting with having one of the dogs keep her company. Being around another living creature immediately stopped the kitten from crying, it worked like a charm with our eldest.

Then my partner put Davion in the room with her.

I haven't been able to watch the camera footage myself. I don't know if I ever will. But my partner has seen what happened beforehand and told me that the kitten was cuddling with Davion and affectionately walking underneath and around him. There wasn't a fight or an obviously hostile interaction leading up to it. Then Davion suddenly bit her. Intentionally. It was no mustake.

It seems to have been a single bite, but she was tiny and the damage was catastrophic. She started bleeding from her nose/mouth. He rushed her to a vet, but her lungs had been punctured and she died. I genuinely cannot comprehend that sentence yet. She's gone.

If either of us had believed Davion represented a danger to her, we would never have left them together. We chose a dog who had spent years demonstrating that he could coexist safely with cats. Hell, once my eldest felt into freezing river and i jumped immediately to save him.

And now I'm struggling with something beyond the question of why the attack happened. I can't look at Davion the same way anymore.

I know intellectually that he's a dog and that assigning human morality to his behavior doesn't make sense. I know he didn't sit there understanding the consequences of what he was doing in the way a person would. None of that changes my emotional reaction. When I look at him, I think about her. I think how she could have been alive and enjoying life if it wasn't for him.

I'm beyond angry at him. I'm disturbed by him. I resent him. I don't want him anywhere near me. I don't know whether I'll ever be able to forgive him, if "forgive" is even a meaningful concept here.

This is particularly painful because Davion was such an important part of our life and our future. We're getting married, and he was supposed to bring us our rings at our wedding. I was imagining him being part of that day. Now, only a day after this happened, I'm disturbed simply looking at him. I can't reconcile the dog I knew and loved with the fact that this same dog killed our kitten. The same dog that always knew when he overstepped in olay and was gentle towards everyone. We got the kitten cremated. I'm crying rivers every single day, even at work. I've lost 2 pets in a few seconds and that doesn't even count my other dog that's terminally ill with cancer and is already living on borrowed time.

We're considering having him stay with my parents for a few weeks because I don't know whether being around him while we're in this state is good for us or for him.

I suppose fostering is over for us as well and that was a big part of my life. Do I grief the entire part of my personality.

I'm desperately trying to understand what actually happened behaviorally. Can a very active, fast-moving kitten suddenly trigger a grab/predatory response in a dog that has lived safely with cats for years and has never previously shown obvious prey drive toward them? Could a single bite-and-release instead be an attempted correction, startle response or play interaction in which a large dog used catastrophically inappropriate force? Is there anything about the pattern that helps distinguish these possibilities?

And then there's the question I don't know how to answer at all: How do you live with your dog after your dog kills another animal you loved? He's medically ok, vet doesn't see anything wrong with him from the outside and said it's pointless to do tests of any sort because it's just animal behavior.

If anyone has experienced something like this, did you ever manage to feel affection for your dog normally again? Did the association between the dog and the death eventually weaken? Did you permanently change how you felt about the dog? Did you ultimately decide that the dog needed another home?

I'm trying to understand how something that seemed safe based on years of experience could go catastrophically wrong in seconds — and whether it's possible to have a relationship with him after something I currently cannot imagine ever making peace with. I'm leaning closer to rehoming although it breaks my heart to million pieces...