If Bitcoin taxes the government or identifies people who do money laundering, then will be the get legality of Bitcoin?
There are 100s of centralized payment gateway for this, it will just turn bitcoin into another payment processor out there which can ban/freeze or even charge heavy fees on transfers.
Bitcoin trading can still be taxable and many governments have their own regulatory terms for this, but there are still many countries out there which either don't have enough man power or technology to regulate bitcoin or they simply are too afraid of bitcoin taking over whole financial system.
Bitcoin's success is more in its store of value aspect rather than paying for goods nowadays, its like digital gold which is untraceable and with finite supply. Many countries add tax on it just like in other commodity. Legality only matters if you want to exchange large amount of bitcoin through regular bitcoin trading/exchange platform.