Thank you for the additional information.
I am happy to hear the thyroid was checked and ruled out as the cause.
You mention he was having some seizure activity and the sudden aggression may actually be an extension of that. Seizures manifest in different manners and a Psychomotor seizure can cause sudden aggression. LINK
If he is having trouble seeing then this to would account for the walking in patterns, clumsiness, aggression( easily startled) and going in and out of the doors. Some dogs do great when they start loosing their sight others do not.
All of that said yes this can also be behavior seen with a brain tumor and the only way to know that for sure is to have an MRI done.
As for when to let him go that has to be a personal decision. I know it is never easy letting go of a friend and e would never want to hurt a family member on purpose.
Unfortunately what ever this is altering his personality to the point that he is not safe and for that I am sorry.
No one would disagree with you in regards XXXXX XXXXX go and sometimes the last act of kindness we can show our pets is to let go so they too do not suffer.
I wish I could give you more positive news. Please reply back if you have additional questions and I will be happy to continue.