Does the rescue you work with allow you to make the decision to put down the dog? Around here, the dog "belongs" to the rescue. They have the final decision whether or not to put the dog down.
It sounds like you need a very experienced dog trainer for him.
Rescues are not like puppies that come with a blank slate. They usually come with severe emotion/behavior problems. I think a competent trainer could "fix" him. Even a change in environment might help. It sounds like he is spending a considerable amount of time in your room or being crated.
If your previous dog had aggression issues, I would think there is something you are doing as a dog owner that might be contributing to this. I haven't seen you handle the dog, I don't know how you discipline (or don't discipline the dog) but I've seen plenty of owners with supposedly "vicious" horses that were making mistakes contributing to the behavior.
If you are going to continue to rescue "problem" dogs, you need to find a competent trainer to help you with these issues! Aggression is common in dogs (they are predators remember), and you will face these types of issues again!
Watch this trainer deal with aggressive dogs (notice how much the first dog improves by the end of the video).
If a territorially aggressive dog is allowed to threaten visitors and they subsequently retreat, the dog will see their withdrawal as a reward for aggressive responding. This will cause the unwanted behavior to escalate.
An article on territorial aggression: TERRITORIAL AGGRESSION DIRECTED TOWARD VISITORS
Quite frankly, I think you are out of your depth with this dog. It does not sound like you have the training knowledge or resources to deal with this behavior. If you do not want to put the dog down, there are a few things you can do: 1) enlist the help of a trainer 2) get the dog neutered and 3) start dog obedience classes weekly and 4) work that dog hard! Sitting in a crate all day gives the dog plenty of frustration/energy build up. A 20 minute walk around the block is not enough. He needs 2 hours of exercise a day...
I might be tempted to start hitting people if I was locked up in a jail cell all day for no reason. Any animal that is locked up/crated/stalled is not a happy animal. Some handle it better than others, but being crated is extremely stressful, and as that stress builds so does the dogs aggression.
The stallion at my barn took a chunk out of the barn owner the other day... Too much energy from being stalled, not enough turn out, high testosterone levels... Should we put him down too?
Unfortunately this is not a "natural environment" for a dog. I bet you would have a different animal if you gave the dog 10 acres to run on in a pack with other dogs. If he showed "territorial behavior" the pack leader would put him in his place.
Obviously this dog is not going to be good with children, but there are plenty of farmers where I live who would desire a dog like him!
I think you need to take a close look at how you are managing these foster dogs of yours. It doesn't sound like a natural environment for them... Maybe in the future you need to stick to small house dogs. Big dogs need a job, with lots of miles to run! Small dogs are easier to wear out. I still hate the idea of crating. The dog only gets a few hours to move around, then it is bed time.