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.
Well this is my thinking, I'm not saying Bitcoin would die completely and lose the dedicated user base that it has built so far. I just think that it would not be able to grow and stuck here, as what it is today, if governments decided that's what they wanted to happen. A lot of people on this forum don't understand, or try to understate the amount of power that the government really holds.
If every country really started cracking down, sure bitcoins would be put into a choke hold VUT that's not going to happen because so many cou tries fully support it. Even if bitcoins where a japan only currency, they would still hold huge value. Keep them no matter what happens in your county.
Ok this isn't true, no country supports Bitcoin.