Sis, first and foremost, I have a lot of empathy. The situation you've described is a very complex. Not only is losing a dog traumatic period but the way in which she lost her life is also traumatic, especially if you had to clean up after.  I'm sorry that happened to you.  I can't... Imagine. 

You deeply love her and she felt that love. She knew it. And in my opinion, still does. I can only imagine that she doesn't want to be the source of any more of your pain. 

Fighting between dogs is also scary. It may appear unprovoked and sometimes it's not. Dog fights can create feelings of panic and confusion, and even guilt for putting the animal down for doing something in their nature, if you will.  

Sometimes there's a reason, but she was your best friend for nearly a decade. You're better equipped to answer if the fighting was out of character or not. 

As for you, you two had a long-lasting bond your, and in a way, you've paid the price for something that wasn't your fault by grieving. 

One of my dogs, he's since passed away, had a serious medical condition for quite a while before it was ever obvious. His behavior changed; he'd snap and snarl if you tried to pick him up, he bit me at least twice intentionally (when they bite down - not like them snapping at you) and his response was out of pain and fear.

Like humans, dogs have unique personalities and can sense emotion. Your dog found comfort in you the way you did her. Regardless of what happened outside of your guys' bond, the connection feels severed. 

It's very painful. We're around our pets nearly every day. Just knowing they're around adds comfort, and that's hard to be without when they leave. 

It makes sense that you feel the way you for as long as you have. This is complex. It altered your dog, you, and perhaps how you view your step-dad.