We went thru the same thing a little over a year ago with our 18 yr. old
poodle. He was growling at everyone, completely miserable, could barely walk. We didn't put him down, but let nature takes it's course and if we had it to do over, I would have put him to sleep:

As he suffered so much in those last few days, he never got better. So as hard as decision as it is, it will only get worse, not better.
So sorry you are going thru this, our thoughts are with you.
Martha