The legality of bitcoin depends on who you are, and what youre doing with it.
There are three main categories of bitcoin stakeholder. Someone may fall under more than one of these categories, and each category has its own legal considerations.
I think the legality will not depend on us like if we will use it in legal and illegal activities but it will fully depend of the nation or the government as the lead rule on how the Bitcoin will be use. If the government will declare it legal then of course regulation will be strictly to be implemented to prevent us of using it illegally. If the government will declared it illegal then for sure no one can use it either in legal and illegal means.