how about the tax for cigarettes?
The cigarette tax may be morally justified as a way to reduce smoking and save lives. According to the CDC smokers tend to live around 14 years less than non smokers.
However, I've read that the justification that "smoking cost society money" is not factually correct. Premature deaths due to smoking tend to occur earlier and more rapidly and thus are cheaper than supporting someone into their old age followed by a long hospital stay before death.
And what about the people that consume soda in moderation and in addition to healthy food? I have soda every day. I also exercise every day and am in very good shape. Not everyone who consumes soda is obese or diabetic. If you really want to punish someone, why not do it in a way that doesn't affect everyone else? If you want a "fat tax", why not do it based on the health of the individual?
For the middle class, this would be more optimal than taxing soda, since it rewards the outputs rather than the inputs. For the poor (who mostly don't pay taxes anyway), this wouldn't work so effectively.