Going from football to boxing, just so you get some pain for letting them lose money, and it's funny.
It's a game; one can lose or win, but these people do not want anything other than winning. Sometimes it's a way for them to console themselves after the loss. This also happened in football; despite the love they have for you, you're the cost of their loss. For that day, you automatically become a careless baller or keeper. I see no reason for being aggressive about not winning, and the statement isn't called for.
There are people who blame them secretly and they won't know it, so they won't feel bad about it but even if let say they are aware, I still don't think they will beat their selves for that. They are professionals and it's not their fault already but it's the fault of the gamblers because they may be too greedy, lazy, ignorant, etc... to try and earn money out of gambling.
I would have said gambling is the cause for this, but it's wrong. Thinking about it, before the institution of gambling in a universal usage across the world, I grew in an environment where people watched sport without gambling and still exhibit similar behaviors like destroying things or really get sad, because their team lost a penalty kick. Gambling I would say brings out the true behavior of some people regarding activities like sports.
Because, these people who behave this way were groomed to behave in such manner. Remember not all gamblers does a things as cruel as that. Hence, players could be aggressive naturally, and whenever something gets them sad they'll begin to act aggressively. Gambling is a factor that makes such people sad to an extent of causing troubles or become destructive. Gambling, yes, can influence someone to behave in such a way. Yet, the whole blame should be on gambling, as the cause. The player also contributed to the way he acted.