Taxation can only make the price of Bitcoin higher. Taxation means that the country officially recognizes that the benefits of Bitcoin are legitimate rights and interests. Since paying taxes in accordance with the law, the country has an obligation to protect. 
I don't think so because the taxes should only be applied if you are going to sell it meaning that the prices are going to go down when there is taxes and probably the prices will go down much more since buying bitcoin might get taxed too.