I always considered it a mix. I mean I think genetically some people are more predisposed to be obese. Which means they typically have to work twice as hard to lose or maintain their weight than most people.
Then there is the factor of environment. How your parents raised you to eat or not eat. This is an incredibly difficult thing to rewire your brain if you have been basically "trained" to eat badly. Not to mention the impact your friends have on your eating while you are growing up.
Obviously, you still have control over yourself. But fighting genetics is incredibly difficult and so is rewiring your brain.
I think the idea of fault is damaging to a person's psyche. It's better just to not think of it as your fault, your parents' fault, or anyone else's.
Just decide that fault doesn't matter and just do what you need to do to get healthy.