For a start, what you did for your friend was one of the bravest and most noble things I've heard of. The fact that you decided to accompany him to his transition from this world to the next was by far the best thing you could have done under the circumstances. He went happy, and peacefully, and no-one can be as true a friend as that.
However upsetting it may be for you to hear this, you mustn't forget he was one of the more powerful breeds man has manipulated, and with his genetically engineered strengths, has become something of a Frankensteins Monster. Something which it's creator undercalculated, didn't understand, and feared. He obviously had no vicious streak in him, but everybody has a breaking point, and unfortunately he was pushed to his and struck out, quite probably only to defend himself, but struck out nonetheless, and to all and sundry it was seen as an attack.
Whoever goaded such a powerful creature to such a point strikes me as a thoughtless or inconsiderate person(and probably deserved their injuries), but the law is the law, and once he struck back, his fate was sealed.
For your partner, please don't tar her with the same brush as you do her sibling. It more than likely hurt her two-fold, seeing a family member physically attacked, plus watching partner having to destroy the thing he loved. GIve her some credit, cut her some slack.
Try sitting down with her and talking through everything, including your bitterness at the whole incident, but be extra vigilant in reassuring her of your feelings for her, and of her endurance and coherence since then in your relationship.
I know how much you must hurt.
I once lost such a friend too. Billy, a golden spaniel, who again was raised from a pup. He came with me everywhere, even after he'd been hit by a car he came running, limping and crawling to the roadside just to get to me after it happened, and he survived months after that.
Being a dog, he barked and growled a lot as we played and teased around my backyard, which irritated our neighbour at the time.
I came home from school one day, and he was lain on his side in our garage, a tiny trickle of blood from his mouth. The neighbour had given him strychnine on a piece of meat just before I got home, he was still warm to touch. I probably missed saving him by less than an hour.
In a fit of temper, I lobbed a large stone across our 6 foot dividing wall, smashing his 5ft high by 10ft wide bay window. He knew it was me, but had no proof, so I revelled in my crime.
I'm not suggesting you find a large rock or anything..........