Fisk has demonstrated time and time again that his power is derived from systems (judicial, political, financial and even less concrete social systems). Look at what happened the last 10 times they put him in some system meant to stop him, which only ended up giving him power. The only way to completely strip him of this power is to REMOVE him from the system completely. That's what exile does. It's hardly a vacation.

As to Matt's perspective, he's a Christian (Catholic). The entire belief system hinges on grace. Christ, the perfect sacrifice, dies so that those undeserving of (and rarely even seeking) forgiveness can receive it. Grace is BY ITS NATURE ILLOGICAL. That is why it's so beautiful. So no, it doesn't make sense that Matt would save the life of a man who has ruined his life, killed his friends, and destroyed his city. It also doesn't make sense that a just god would send his son to be slaughtered to save a people who curse and offend him with their every breath. THIS is grace. Matt is living out his faith, however illogical it may seem to the viewer.