Helping Others Who Still Need to Suffer due to karma
Karma is not meant as a punishment or retribution or debt to be paid which you are not to interfere with. It is complicated with many interactions. The environment has to fall into place for its results to be seen. 

If the environment can influence the effects of Karma, it can be eradicated or changed in some cases. Since Karma is complex you you cannot say for sure.

By helping others your can help lighten the burden of their Karma and in some cases counteract the bad Karma altogether. 

Also this is the good Karma for the person doing it.