Insulin is a hormone made by the body to shuddle nutrients to tissue. It does not only act on muscle tissue. It causes growth in all tissue ie; adipose fat tissue, smooth mussle tissue (intestines) cardiac muscle tissue. If you take it and your diet is not 100 % on, you will only get fat, or worst case scenario, you will die. Body builders generally use fast acting insulin until that insulin abuse causes insulin dependence. At that point you will be considered diabetic. The fast acting insulin is used one of two ways, per workout or post work out. Used preworkout without timing your preworkout meal and intra workout nutrition correctly could cause low blood sugar. Have you ever worked with someone that has low blood sugar? They can't understand what you are telling them, they are confused and sometimes become violent. Low blood sugar can lead to diabetic coma.
Used post workout the insulin is used to shuttle post workout nutrients to damaged mussle tissue that soaks it up like a sponge. Combined with the effects of aas on mussle repair, this is a great combo. A common mistake is to workout in the evening, get the post workout meal, and before the insulin has had time to be used up, the person goes to sleep. While asleep the blood sugar drops, the person slips into a coma. This can be fatal.
Insulin is the most dangerous drug used in bodybuilding. It should be the last drug added to your stack. This drug is only for advanced users. You weigh about 170 lbs. Until you hit 230 lbm and have proven you can time diet and drug intake correcly and understand the dangers of abuse, you should not even think about trying insulin.