After a few days of betting, player called "zhent" managed to somehow win 200 BTC at betking.io What are your thoughts how is it possible some people manage to win so much? Just a matter of luck or is it the bankroll that is the key? It's very difficult to win with a very limited bankroll and it's coming from experience 
A post of the winner on BetKing thread: https://bitcointalk.org/index.php?topic=910134.msg13880555#msg13880555
Both luck and bankroll are deciding factors for how much you'll win. Obviously, if you have a large bankroll you have more chances to win, and it coincides with your potential profit. Though, the problem is, your risk of ruin increases... A post of the winner on BetKing thread: https://bitcointalk.org/index.php?topic=910134.msg13880555#msg13880555
Do you really want to risk your entire bankroll on these bets? Jeez. Most likely, he gambled pretty seriously and he might have bet extremely high amounts. It's incredibly risky to do that.