The first question is if the bitcoin if the blockchain suffer 51% attack, the bad miner will double spend some bitcoin. If the network is back to normal, will the coin that was double spend be reversed and everything again be correct to normal?
No. Once the attacker overtakes the main chain with their evil chain, then the evil chain becomes the main chain by virtue of it having more accumulated work. All the honest miners would swap over the evil chain (which is now the main chain) and the old main chain would be discarded. Any transactions present in the old honest chain and not in the new evil chain would either be returned to the mempool as unconfirmed transactions, or would be invalidated if one or more of their inputs had already been spent.According to what I read, that the bad miners will have their own chain. This means that there will be two separate blockchain, one belonging to bad miners and the other one which is the original blockchain, or there will be different chains on the same blockchain, one for original miners and the other for bad miners?
It depends. What the protocol says is that all miners would swap over to the chain with the most work. There could, however, be a scenario where some of the miners/nodes/user refuse to do that and fork the coin in to another chain. For example, see ETH and ETC. While not a 51% attack, ETC was created by some users who disagreed with Ethereum exercising centralized control over which transactions were and were not "allowed" to be included.that 1 confirmation transactions can be reversed and be double spend by the bad miners?
It's not as simple as that. Although it is certainly easier to reverse 1 confirmation as compared to 6, say, 1 confirmation is still secure enough for the vast majority of transactions. By the same logic, it is easier to reverse 6 than it is to reverse 20, although in reality neither of these will ever happen. 
