Connolly and Morris may have come in with noble intentions, but as the years pass their deal with Bulger becomes increasingly corrupt. In fact, there are multiple instance when they leak intel to Bulger that he uses to kill people. That's well beyond fudging reports or inflating your informant's credibility. That's being an accessory to murder.

Connolly might not be a good FBI agent, but he's a pro at forging documents. He's, like, the Michael Jordan of subverting the truth and exploiting loopholes in regulations. Hooray?

Morris resists for a while, but he eventually gives in and starts taking straight cash from Bulger and the gang. This is his personal Rubicon: he can never go back to being a normal FBI agent now.