[....] what would happen if someone would win 100 x 20 btc (and investments would all be negative) he just said : "we will not pay him the won amount" [...]
Can you please provide evidence of this. Perhaps a skype screenshot?
If this is true, this is extremely serious. By accepting bets they knowingly can't or won't pay, they are basically scamming users.
If those replies are from the real dadice team then I don't know if this would end up a good story..