You seem like a very caring person. Your concern for this unfortunate animal’s life does you credit. Sounds to me like this Lab-Mix is unstable to say the least. The thugs who have abused this poor dog, and ultimately cost took the life of your beloved pet, need to answer for their crimes. Sad as it is, maybe calling the proper authorities is the best thing to do. I don’t know much about Great Britain’s legal system as it pertains to animals, much less any local laws in your area. As painful as it is for me to say this, your local animal control professionals might be the best option.