In a 1970s organized-crime trial, the late Norman Ostrow cross-examined Jimmy "The Weasel" Frattiano, a government cooperator who had admitted to a number of grisly murders.
Add to that a situation where the cooperation is in the context of an unfair persecution of a friend, and the moral standing of the cooperator becomes even lower.
However, if the cooperator is culpable in the horrific behavior, it is hard to have sympathy for the cooperator simply because he testified against his partner in crime in exchange for a lighter sentence.