It isn't easy work. If a person is in the trenches they need to keep some ease of heart or they either burn out or callous over. If the first happens, we lose good people when they quit. If the second happens, good people become ineffective.
There are a number of things that help people keep they eyes and hearts open while they watch and address suffering. One very simple solution is to sit everyday in meditation. Dr. Sara Lazar has conducted research that definitively proves that the brains of meditators change in many ways, and specifically changes in a way that increases the capacity to feel compassion. (https://www.nmr.mgh.harvard.edu/~lazar/)
Meditating activists feel compassion not only for their clients, but also for the people causing the problems being addressed. This doesn't mean the offenders get off the hook. It means the workers don't get consumed by anger.
Compassion most profoundly affects the people experiencing compassion. It's just easier to keep facing dark realities when you feel softhearted instead of angry.