Here's David Simon on why they had Suiter commit suicide instead of having him murdered in order to silence him, which would have been the better dramatic choice.
Working on the scripts for a six-part miniseries on Baltimore’s Gun Trace Task Force scandal, the writers inevitably arrived at the moment when Detective Suiter’s life would end. Certainly, the best narrative for our purposes – as dramatists chronicling a world of corrupt policing – would have been one in which officers involved in those corruptions decide to murder one of their own. No other version of events could deliver as dark, disturbing and provocative a verdict as to imply that in the end, those at the core of the scandal were capable of betraying a comrade to save themselves. If we serviced our miniseries as an entertainment only, we could film the legend and relish the best possible ending to our tale.
Instead, we did what dramatists are not required to do, but that which should be demanded of writers undertaking to depict a true story. We looked at everything dispassionately, carefully, and independently. We consulted all that was known and credible, talking further with critical principals in the investigation to elucidate points. Eventually, we came not to the best and most exciting conclusion for our story, but to the the most pragmatic and comprehensive truth, which is there in plain sight for anyone who still wants the truth. Sean Suiter killed himself, attempting – and indeed partially succeeding – at making his sudden end look like a line-of-duty death.
Sean Suiter – The Audacity of Despair (davidsimon.com)