He wanted to convey that indifference is worse than hate or anger. One could be angry at injustice or hate evil, violent acts Indifference is the absence of compassion and implies something worse than outright hate; indifference implies a lack of acknowledgment. Being indifferent to another's suffering is like saying, 'you're suffering is not even worth my consideration.' Wiesel speaks from his experience of the Holocaust, but this could be applied to any situation in history in which the world was indifferent; in which the world willfully refused to acknowledge suffering of others for any number of unjustifiable reasons: 1) out of sight, out of mind, 2) passivity, laziness, 3) an untried feeling of hopelessness ('what could i possibly do?'), 4) selfishness. When Wiesel speaks of indifference he also means ignorance in 3 senses: 1) ignorant as in lacking sensitivity, 2) lacking knowledge and 3) ignoring. The 'perils of indifference' could be described as the 'the terrible outcomes of ignoring atrocities. Apply this to anything today, where suffering is ignored by indifferent people and governments. (i.e., Darfur, Haiti). The peril of indifference would be to allow (allow by ignoring = indifference) an atrocity like the Holocaust to occur again.
