They couldn't kill it completely, but they can prevent mass adoption by simply outlawing it. Most people wouldn't risk getting fined or imprisoned just to use an alternative payment system, so Bitcoin community wouldn't be able to grow to any significant size. And the second problem is mining - right now it's relatively centralized and draws huge amounts of electricity - together it makes it easy to shut it down. Without mining Bitcoin's price can't freely grow, because mining protects the network from double-spending attacks, so when hashrate is low the network becomes less secure, meaning it's cheaper to attack it, so it can't sustain above certain amounts of value.