It is very simple. Just tell them that Bitcoin and fiat money is almost the same. It can use in good deeds and it can also use in bad. If you use US dollar in illegal activities, is that mean that the dollar is bad?
of course not, all depending on where the bitcoin it used, that will establish the true nature of the bitcoin. but the negative news is always evolving faster than positive news.
We can't force people to believe in the bitcoin as currency, because reality itself is still a lot of people who consider bitcoin as merchandise traded in the market not as a currency used as means of payment. Although some can already use the bitcoin as money, but the percentage will still be more that considers the bitcoin as investment assets.