I don't think it will illegal.
depending on where you live, in my country they 'government' gamble with bitcoin it is illegal and unlawful. But do not worry that many countries are offering bitcoin bets and their governments legalize bitcoin bets and can be used for betting.
Yes sir i agreed with you, it depends upon on which country you are if you think that bitcoin will illegal at your place then also gambling is illegal too..Besides there are some countries now embracing bitcoin and they agreed on using bitcoin as deposit an gambling.
So, i think in betting using bitcoin is not illegal if they recognized bitcoin to their country lets just wait.