Despite intensive research, the causes of the obesity epidemic remain incompletely understood and conventional calorie-restricted diets continue to lack long-term efficacy. According to the carbohydrate-insulin model (CIM) of obesity, recent increases in the consumption of processed, high–glycemic-load carbohydrates produce hormonal changes that promote calorie deposition in adipose tissue, exacerbate hunger, and lower energy expenditure. Basic and genetic research provides mechanistic evidence in support of the CIM. In animals, dietary composition has been clearly demonstrated to affect metabolism and body composition, independently of calorie intake, consistent with CIM predictions. Meta-analyses of behavioral trials report greater weight loss with reduced-glycemic load vs low-fat diets, though these studies characteristically suffer from poor long-term compliance. Feeding studies have lacked the rigor and duration to test the CIM, but the longest such studies tend to show metabolic advantages for low-glycemic load vs low-fat diets. Beyond the type and amount of carbohydrate consumed, the CIM provides a conceptual framework for understanding how many dietary and nondietary exposures might alter hormones, metabolism, and adipocyte biology in ways that could predispose to obesity. Pending definitive studies, the principles of a low-glycemic load diet offer a practical alternative to the conventional focus on dietary fat and calorie restriction.
[Sorry to keep reposting this, I've had to move communities 3 times in the last month, it makes linking references a chore]
I think it's unusual to say that insulin is bad. Chronic insulin resistance is surely a sign of poor metabolic health, because it means that the body has had to secrete a lot of it to address persistently high levels of glucose in the bloodstream, which is caused by eating too much sugar in any of its forms. Sugar (and by extension carbohydrates) is what is bad.
Like you said, it's a critical hormone and we cannot function without it. T1D is an inability to secrete enough insulin and those affected waste away very quickly without medical aid.