Complex sentence: Believing that Coonan had deliberately framed him, Featherstone became an informant and testified against fellow Westies, including Coonan, and during the later trial of John Gotti.
Split into simpler sentences: Believing that Coonan had deliberately framed him, Featherstone flipped and became an informant. He testified against fellow Westies such as Coonan, and later at the trial of John Gotti.