Have to remember that Natasha doesn't just compete against all the other players, there is a fierce family rivalry also. Goes back to the days of their late father John Connors who founded the GMC™ almost 20 years ago.
They are very nice people, the nicest you could ever possibly meet, but when they play against each other it is all out war with no holds barred. The sole aim is to kill everyone who gets in their way at the poker table and that includes each other! They are all great players and so of course was their father.