Yes, you're right, even though I'm also a Barcelona fan, when their performance declines, I'll choose to bet on another, stronger club.
What this gambler is doing isn't betraying his favorite club; rather, this bet is about the financial rewards we win and lose.
After all, even though we're betting against our favorite club, we still support it and hope it wins.
In my opinion, anyone who bets emotionally is simply an overly fanatical fan who considers betting on their favorite club to be the same as support for that club to win.
Moreover, I will say here that in the case of betting, there is no such rule that if we bet against our favorite team, it will be betraying our favorite team, because no profit or loss of that team depends on our betting. However, if we become a little too emotionally attached and try to protect our emotions and bet on our favorite team even though the opposing team is strong, then even if that favorite team does not gain or lose, if they lose, we will lose.
So, it is better to refrain from making such bets without wasting unnecessary emotion here. And the team that logical analysis supports should be the team that is being picked.