I mean that is possible, but branding someone a rat when there is no concrete evidence is something I am never a fan of.
But they are already proven drug dealers and murderers, being branded as "rats" would be just a minor addition to the already definitely compromised reputation. While I can understand that informants are usually scumbags, but that's because they do so to put the blame for the crimes off themselves and on somebody else. If they did testify out of genuine repentance, there would be nothing dishonorable in this. While murder committed out of power and money is always a shitty thing.