I think is what the author is trying to say is that it is better that there is a God who cares than one who does not care at all.
The word "unjust" means that sometimes things happen that cannot be justified. Bad things happen to good people. We do not know why God has chosen to take someone away from us or let bad things happen to them. To us, it is very unjust.
Indifferent simply means that it does not make a difference either way. It implies that God doe snot care. Even if we cannot understand why God has chosen something, he has done it for some reason.
God offers serenity to many people. Faith is also a very important concept here. People who believe in God have faith and they trust that whatever decisions God has made, they are just.
What Elie Wiesel is saying here is that it is better to have someone (in this case God) care about you and be present than to have them not care. This is true, he says, even if God is unjust. So he is saying that it is better to be cared about by an unjust God than to have a God who just does not care.
In this part of his speech, he is saying that they preferred to have God near them, even if he was unjust. What they thought would be the worst is he if God abandoned them altogether.