There is no known antidote, but there is a treatment procedure. If bitten they put you on artificial respratory and heart machine until about 24 hours. After the poison works it's way out of your system most make a full recovery. The octopus venom is more dangerous to children and adults with low body weight, but can kill any person it bites.
As far as keeping the animal, as long as you know the dangers and take precautions to minimise your exposure it is up to an individual to decide. I have been a collector of animals all of my life, I used to keep and breed venomous snakes (my favorite was cobras) and all of my adult life I've kept big cats (my favorite is a serval which is small, but I like them) but I understand what the potential dangers are. I keep these animals because, just like some of you who love octopus, I am fascinated by the group as a whole. I understand the problems, the animals are not on display, they are caged so even if they get out of the main display they are isolated and can't just get out into an area where they can be a danger. The people who keep them to be able to say I have a cobra, wanna watch it kill something, are the people who AnimalMother was talking about winning a Darwin award. Most of my snakes have never been seen by anyone who isn't in the hobby or doesn't have a working knowledge of what they are looking at, it just isn't worth the risk.