Just because you lack will power, that doesn't make you a bad person. Everyone lacks will power when it comes to something.
For example, I don't know anyone who always drives under the speed limit. Not a single person. Everyone I have ever met in my entire life, and in any culture in any country, has occasionally driven faster than the speed limit. If we're being honest, most of us would have to admit that we exceed the speed limit pretty darn often.
It may seem unusual to think of speeding as a lack of will power, but what else is it? We all know what the posted speed limit is, and yet we choose to exceed it anyway. We all know exactly what we'd have to do in order to avoid speeding, and yet we don't do it. In the end, we don't care so much about the posted speed limit. We're normalized to a world in which exceeding the speed limit is common and socially acceptable. So we don't have any incentive to be the one person on the road who adheres to the posted speed limit sign.
Since this is true, we ought not cower away from saying it like it is: none of us has the will power to consistently obey the speed limit. It doesn't make us bad people, it just means that we ultimately don't care about speed limits. That's just fine.
Unfortunately, when someone makes the same point about obesity, the world objects. If I were to say that obese people aren't bad people, but they simply lack the will power to be thin and healthy, many would start talking about the supposed "addictiveness" of sugar, or the hormones of obese people who lose weight, or the notion of body-shaming, or etc. etc. I think these objections only serve to defend against the charge that obese people are bad people.
But obese people aren't bad people, even if they do lack the will power to be thin and healthy. So, rather than object to the notion that they lack will power, we should embrace the inherent truth of it. Every obese person knows what it takes to lose weight, and to keep the weight off. Everyone who has ever gained weight knew full well what eating all that crap and never exercising was going to do to their bodies. They know, and they don't care, because they don't have the will power to fight against it any more than they have the will power to obey the speed limit consistently. And, just as they are not bad people for speeding, they are not bad people for putting on weight. But that doesn't mean they have will power, either.
As with speeding, obesity is so common that we now live in a society that doesn't much care about it. Obesity has become so normalized that it's no longer discussed using the language of will power and self-control. Why practice self-control, anyway? You can still be a good person, even if you're obese. So, really, why bother?
Well, there are many reasons why a person should aim to be healthy, but I think many adults underestimate just what is required to avoid obesity. One either has to be willing to eat very small meals and seldom snack, or one has to exercise almost constantly, or both. It's painfully obvious that the average American has no intention of living this way any more than the average American intends to follow the speed limit. It's a lack of willingness to do what's necessary to achieve a particular goal, plain and simple. A lack of will power.
It doesn't mean you're a bad person, but it is what it is.