Watch me cram two episodes worth of ideas into a single paragraph.
"If you can't change the world, then you have to change yourself!". This is the line that pushes Shimada over the edge. The smashing of the pucks is a symbolic act of rejection, a rejection of the detective's world view and, as a direct parallel, of the system currently judging them. We are presented with a system so inhuman that nothing less than its complete evisceration at any cost will suffice. Why require victims? Why require "the darkness of the soul"? It is not that Decim is unfit to judge people, the system itself is unfit to judge people. The concept of judging a person's life, itself, is questioned and, along with it, the fairness of the concepts of heaven and hell.
[Subject to further writing if I feel like it but, as of now, I feel like the single paragraph above encapsulates my thoughts on it well enough.]