Why do they say R-12 is bad for the ozone, when it is heavier then air and goes down not up when released?

Answer 1

Because of what it does when it breaks down.
The current scientific theory states that it is because of Freon's eventual effect on the ozone, high up in the earth's atmosphere. The family of Freon type gasses work their way higher and higher into the atmosphere due to kinetic reaction with the other molecules in the air. When released to the atmosphere Freon gradually rises up, even higher then the ozone layer. Once it is higher than the ozone layer it is no longer protected from ultra violet light. Ultraviolet light acts on Freon by breaking it down to its original components. One of these components is chlorine. The liberated chlorine then starts falling back down through the ozone layer changing ozone molecules back to oxygen. Worse yet, what happens on this molecular level is that just one chlorine molecule can destroy millions of ozone molecules on it's way through.

Answer 2

R-12 will disperse into the atmosphere due to wind and thermal current causing reactions with ozone molecules. O3 is also a heavy gass but is dispersed in the same way. It is in the upper atmosphere where ozone is most effective in shielding from us solar radiation, it is also where O3 thinning is most noticeable due to increased radiation levels at ground level.