Are you suggesting that you simply like hurting people and seeing the opportunity to do so in this scenario, you would capitalize on the already occuring violent behavior of others? If that is the case, why take on the role of helping the victim?
And since you suggest you'd leave the victim if you came along after the violence, what if you were successful at causing the gang to disperse and victim was left there hurt. Would you still offer no assistance?
On a side note, do you participate in Mixed Martial Arts or Boxing or any sanctioned sports that involve fighting or do you merely enjoy brawling spontaneously?
In regards to helping those in need when no immediate reward is visible, it can sometimes be the case in my experience that doing acts that involve helping others does go rewarded although in the immediate moment it is unclear that a reward will commence at some point later. As I am sure you are aware, doing favors for humans can be a manipulative tactic to evoke a desire for reciprocation on the part of the one receiving the favor.