Michelle, I have only just found this and on reading through, the reply by JerseyGirl is virtually the answer I was going to write.
You loved Lyndsey as a Mother would love her baby, even if she was treated as a dog, and the shock alone of her turning on you like that is great, without the stress and heartbreak of having to make & take that final decision. Sadly, I have to say that i think it WAS the right decision, no matter what the reason for her bad temper she was too big and strong to risk anything like that happening again, possibly with more serious consequences for you or your family.
Whilst it will take more than a little while for you to recover from these events I hope that in time you will be able to remember her as the beautiful girl that she was.
Thinking of you,
Carol.