no one should ever do insulin. If you need insulin to get big, you just weren't made to be big!
I wouldn't ever recommend anyone use insulin but it's just another hormone endogenously produced in the body. It's neither good or evil intrinsically. This is not a black and white issue IMO. It depends on how it's used like with all things. Some insulin use by a bodybuilder might even be healthy in some very specific instances. Why would GH, steroid, thyroid, stimulant, etc use be acceptable but not insulin?
One thing's for certain, taking insulin is a hell of lot more "natural" than popping d-bol and shooting tren.