Sick Betta

Tracy

I'm so sorry for you and your fish :( There is nothing worse than having to watch something you love, die slowly. I have used the 'freezing' method before (but with Goldfish). It is humane, because as the water temperature drops, the fish's metabolism slows down. He just goes to sleep. It's hard, I know, but better than watching him suffer. I have had one of my bettas for over 3 years now, and he's not eating much these days. I know he isn't going to last forever, and I am preparing myself for it.

Lupin

You have to do it in a quick manner to prevent struggling. The point here is to prevent the fish from agonizing/struggling as it feels the pain, not the cruelty. That will certainly end its misery pretty fast.

While it seems cruel, it's best to finish its misery off immediately. It's even more cruel if you just let it struggle.
But other methods are available so it's not necessary that you do the third method.
Shouldn't be too painful for the weak fish as it wil slowly be anesthesized first by the cold temp, until it becomes unconscious. And finally, it dies slowly.