This is one of the few arguments Trumpsters made that was actually valid. It's a bit overly dramatic, but the fact of the matter is, if and when government takes over health insurance, they will be deciding, via policy and rules mandates, who is worth saving and who isn't.
It's fair to say that, if you're relying on insurance for your health care, we already have "death panels" - the insurance companies and doctors are already making these kinds of decisions. But their decisions our bound by contract. You know what you're getting into ahead of time and can decide to sign up or not. Ironically, government has no such contractual obligation; they can change the rules whenever they want. And you'll have no option to fire them, or choose another provider, if you don't like what they "offer".