That may or may not be bipolar (bipolar is usually manifested as going between happiness and depression, not nice and violent). Either way, it shows signs of a serious mental illness on your uncle's part. I suspect it may be an addiction of some sort (probably alchohol). The fact that he gives you a generous allowance, in opposition of what is legally assault (generally, in the USA, threats of violence consitute assualt, though some southern states like Alabama may still allow threats as a non-crime), suggests that the magnitude of his problem is severe. Everyone has mood shifts to positions that are opposite of each other; it's just considered a mental disorder when they are this strong.
I don't think that we, as Massassians, will leave you alone until you try to get some help. Talk to SOMEBODY authorised. At the very least, your favorite teacher.
Wake up, George Lucas... The Matrix has you...