Apparently it won't be, since some people here seem to think stealing from its users is a commendable practice.
Seriously, how would you handle the case "someone sends an unconfirmed TX right before the bet ends, TX confirms after the bet has been decided" ?
Refund?
You want to discourage scammers, not have to guess every time whether the request is legitimate or not.
Refund.
If you don't want people to bet right before the bet ends, then simply reduce the potential winnings gradually right near the end. For example, if they bet 10 minutes before the bet ends, only give them 10% of the winnings they would have had if they had bet 100 minutes before the bet ends, and spread the other 90% across to the other winners. If it's 25 minutes beforehand, give them 25% of the winnings. Etc, etc. But if they lose the bet, they lose 100% of their bet.
It's not complicated, and you avoid stealing people's money without good reason.
Again I ask, what is your affiliation with this site?