Lol. 6 year old beta? That's like family.
To be honest, you wanna do it fast? Frying pan or hammer in my humble opinion... Or step on him hard. Maybe get your dad or a friend to do it.
Me? I would flush him. I have no problem with flushing fish. I had 2 Jack Demseys that were eating all my other tank fish. I didn't know they were JDs b/c they looked different than the JDs I'd seen before.
After verifying that they were my tank killers - I flushed them alive. The rest of them is a mystery... Just like the mystery behind the 2 betas and 3 platies they ate.
Go buy a new beta. Sounds like you really know how to take care of one.
I'd get one from Walmart. It's like saving an injured pet. They really don't seem to care about their fish. If you buy a fish there, you're practically saving it's life.
Edit: Don't put him in a lake or pond... I'd rather my fish die in the sewer than become a bullied to other fish or become eaten (highly likely with those vibrant colors).