One kind of "bad" is a moral judgment. The act is defined somewhere as a sin, or some such. Another kind of "bad" is ill effects, of which there are two kinds. Ill effects on other people and ill effects on yourself. Lately our society (in the West mostly, in other places in the world the context and the rest of the related factors are different), first smoking and now obesity have come under intense scrutiny for their effects on the rest of us.
We should also keep in mind that money is made and lost for a lot of people in how these things play out. Cigarettes make a lot of money for the corporations that produce them, and also are heavily taxed (taxes of this kind are regressive - that is they are most costly to the underclasses). Given the cultural forces defining beauty, there is also a lot of money made by companies that sell diet plans, products, supplements and so forth.
The studies on secondhand smoke are not very scientific, although this is not told to us. Mostly this kind of scientific investigation is based upon epidemiological studies (statistics of incident correlations among large populations). This kind of science doesn't really know much and assumes a great deal. It can't actually prove a causal relationship between those who are near secondhand smoke and various disease. All they really know is that the incidents of certain disease are higher when some people are exposed to secondhand smoke.
There are also animal studies, but these too have a problem in that usually the effect on the mice and rats (for example) is produced by the application of a excess of the so-called "dangerous" substance. The reality here too is that there are a lot of assumptions involved and not enough real knowledge. Let's look a little closer at obesity to see if we can discover something.
A doctor friend of mine (here's a link to his book), once said to me in conversation that the real cause of obesity is starvation. The body knows its needs, and our food has become so denatured that it actually contains not enough minerals and vitamins to sustain us. Our food also has all the wrong kinds of fats in it as well. The result is that even though we eat a lot, our body continually tells us we are hungry because our real nutritional needs are not being satisfied.
We are then driven to eat and to eat and to eat, trying to receive what we need from the food, but it is not actually in the food. As a consequence (and body type enters in here), for those people who are endomorphic (goolge it) the excess starches aren't eliminated as with other body types, but stored. So when we look at obese people, we are only superficially looking at someone who eats too much. The reality is that they are endomorphic and are starving for real food, and not getting it through what can be found in the grocery store. Presently they are being made social pariahs - outcasts, for something that is really the fault of our society and its profit driven food businesses, coupled with a complete lack of the nutritional knowledge among our medical practitioners. Smoking is similar, but here the problem is in the soul life (that is, it is psychological), not the physical.
Everyone has stress in their life. We all cope with stress in different ways, according to our basic psychological type (what another age called the four temperaments). Yes, of course, the nicotine in cigarettes is addictive, but what is the payoff? Why does the person seek the nicotine in the first place? If we assume it is just because they once tried cigarettes we will not understand, because lots of people try smoking, but not everyone keeps it up, and fewer still make it a lifetime habit. If we assume there is a genuine psychological need, what might we find if we looked for it?
The four temperaments are: choleric, sanguine, phlegmatic and melancholic. A choleric wants to be a leader, has a strong ego sense and would rather be a boss than a worker. Sometimes such people are called: the nervous type. We also sometimes call them nervy. They go to the front. They go after what they want in a direct way. In a lot of biographies, however, this impulse is frustrated. They want to go to the front, but can't. Their nervy impulse is impeded.
As a consequence they are drawn to nicotine to relax. The cigarette calms them. A worker who is a choleric, but has a sanguine boss (a sanguine can't make up his mind), will always be frustrated by the boss, because the choleric sees how they would do it, how they would make a decision and go for it.
I could say a lot more, but here I just wanted again to point out that we live in a culture that is very much ignorant of a lot of the truth about human beings, and for complicated reasons tends to make some groups the bad guy. Some readers here will have perhaps understood that we can only change ourselves, not the world and not the other guy. But the common impulse today is also to want the world to change, so people think that they can fix the world by forcing other people to change. This is an unhealthy social impulse, because it is rooted in ignorance, and as a consequence will not only not really work, but actually cause harm.
There are better ways to do a lot of things, and the ways to help smokers and fat people is not by social control over their freedom. Enough said,...if you don't get the point, there isn't anything more I can offer to help you find the right direction.