Dog killing longtime housemate and best friend

Trying to figure out if my 7yr old Springer did in fact kill his 13 yr old house mate, a border collie. That's what I can home to Oct 18, last Saturday. Have been fostering a white German shepherd /siberian un spayed female for.the Las 2 wks while.looking for her owner. They had all been getting along fine. My dogs are were both neutered. There had been some minor tiffs. But nothing major. I found out from her tag that she is from Killeen TX. Owner must have relocated to ca central valley area. Now I have been putting up with an aggressive female pit bull who the owners rescued from fighting stock. So she has that in her genes. But she is so aggressive that they have put 4x8 sheets of plywood around the perimeter of their yard. That's how hard she jumps up against the fence. I'm used to it now. But she scared the hell out of my daughter once trying to dig under the fence to g et in my yard. My Springer lines to fence fight with her. But when I'm home, I don't allow it. In addition, when I walk my own dogs together, they're very dog aggressive. But when I walk my Springer alone, he's fine. But the border was in my opinion, fearful aggressive. Always looking for a fight. And not one to back down. And when they can't get to the other dog, they attack each other. So for many years I couldn't walk them together. Since they have gotten older, they've mellowed quite a bit and I've been walking them together with very few incidents and was able to stop it rt away. So that is the history of the situation. When I got home after leaving the 3 of them together in mg backyard, I found my 13 had been so viciously attacked, that he was barely conscious. My Springer had blood on his face and chest. The female only had I on her mouth. The pit next door didn't make a sound. Maybe it was inside. Don't know. There was no sign she got over or under the fence. My Springer wouldn't look me in the eye and he always makes eye contact. She was upset. But acting fairly normal. Will someone please tell me what happened. My dogs were best friends and played together. I know she was a fly in the ointment. But they both had ignored her unless she forced the issue. Then there would be a curled lip and low growl and she'd get the msg and leave. And she's not in heat. Don't know whether to keep her so he will have a companion. Don't know what to do to help my Springer recover. But need some answers. My friend says rule.of.thumb. No more than 2 dogs. And I wasn't planning to keep her. She's listed online. I really need some suggestions. And if someone can tell me other than the obvious, what happened. My dogs have been in the back with the pit yapping at them for yrs. So she must have been some sort of catalyst. Does anyone have some idea of what happened and what I should do now? Bewildered in Turlock.

Far too many dangerous variables to judge what happened.

Your dogs were living under appallingly stressful conditions - in my mind the death or serious injury of one of them was inevitable. I am so sorry for your 13 year border collie what a terrible way to lose her life.

If I took her out of the mix, would u still consider my dogs to be living in appallingly stressful conditions. I cant control my neighbor's dog. I can only control my own. I have now made a barricade allowing 6ft between my patio and the fence nxt door. My remaining dog still wants to go out and fence fight the pit bull nxt door. He is in quarantine now. I normally walked them and they had been getting better about redirectional fighting. Mellowing with age. Or so I thought. There was a lot of info I didn't include because it would be too much. But the dog who died had issues due to being left alone too much do to unforeseen circumstances in my life. And of course it's hard to undo that kind of damage. Before I got him a housemate. They played happily together for yrs. Never ever fought at home. Only when on walks where there were approaching dogs. So for a while I had to walk them separately. Turns out the female was I heat. But she didn't start bleeding to yesterday. So I was a terrible "perfect storm" scenario. The pit next door is female. She could be in heat also. My 2 dogs had a very happy life until I decided to be a good person and help the lost dog. No good deed goes un punished. Story of my life. And now she is with her owner. And I'm without a dog. And that is a story in itself. She and her puppy were stolen for ransom. So it was a criminal case. So had to take the puppy from my grandson. But he's 18 and sad but dealing with it.