Any sin should make us indignant. Problem is discerning the difference between true indignation, which is in the higher faculties and motivated by charity, and the passion of anger. Often these two blur, and even the latter dominates, so that at best we manage a mixture of the two. So it's not so much about the object of our indignation (i.e. WHAT causes us to be indignant) as it is about what's going on inside of us.