I suppose there are a lot of factors here, the sum stolen, the means by which you steal it (robbing the bank at gunpoint would likely not be a very positive act), what you do with the money, etc. I suppose we do look at the Robin Hood scenario of robbing from the rich and giving to the poor as a positive example, but I think that is an example of a different time with different social dynamics. Plus it's just a story 
Also, think about the consequences of stealing from bankers, how do you think they would react? They'd likely be rather upset and that anger would be focused outwardly on the lower classes who they would blame for this, they would also want to recoup their losses possibly by implementing stricter regulations or enforcing more oppressive security measures. If they discover you are behind it you will be marked and spend the rest of your days in jail or avoiding the authorities. Also say you take a large enough sum to destabilize the institution, we've already seen that the government is willing to hand over huge amounts of tax dollars to failing banks. So what would the net result be?
Would stealing from them spread more negativity? I think so. There are more balanced ways of coming across money and helping those less fortunate, as Plenum mentioned a positive entity might do so by generating their own means and then sharing it.
Also, think about the consequences of stealing from bankers, how do you think they would react? They'd likely be rather upset and that anger would be focused outwardly on the lower classes who they would blame for this, they would also want to recoup their losses possibly by implementing stricter regulations or enforcing more oppressive security measures. If they discover you are behind it you will be marked and spend the rest of your days in jail or avoiding the authorities. Also say you take a large enough sum to destabilize the institution, we've already seen that the government is willing to hand over huge amounts of tax dollars to failing banks. So what would the net result be?
Would stealing from them spread more negativity? I think so. There are more balanced ways of coming across money and helping those less fortunate, as Plenum mentioned a positive entity might do so by generating their own means and then sharing it.