If a diabetic cat doesn't get insulin, they die. It's as simple as that. Not to be dramatic but diabetes is deadly when not treated.
The needles used to administer insulin are so small and so sharp that the cat won't even feel it most of the time. So it's probably not the needle itself that bothers him, just the overall process. And this only gets better with repetition. Being that the needle is often undected by the cat the best time to do it is when they're distracted. While they're sleeping or while they're eating. Best thing to do is not make a big event out of it. Just stick them when they don't expect it.
Throughout the day I'd suggest just tugging at your cat's scruff (the thick folds of skin at their neck). This is the best place to give a shot. While you're petting just tug at it and pet it so he get's used to this area being handled. When you give the shot, you just pull up on this area to make a little tent, poke the needle (they're sharp so it hardly requires any force), push the plunger, give treats and head butts, done. It's really not a big deal, just practice.
If you associate this process with rewards, your cat will probably WANT to get his shot, because they want those treats. I had difficulty at the beginning with my cat. My first few attempts were fur shots (needle didn't go in because I panicked, insulin went into the fur). But by now, if I miss giving his shot at the usual time by anything like 10 minutes he'll come yell at me.