You dont get money because someone else lost some. You get money because you helped stabilizing the market by predicting it right. The losers lose because they predicted it wrong. Its a net gain for everyone who is somehow affected by the market which means basically everyone on this planet. There is no moral problem with winning or with loosing as you help either way.

Also there is no need for anyone to lose, everyone could be right all the time that would give a perfectly stable market, but people are terrible at predicting the future so that will never happens.