Yarmouth police said in an emotional post the felon charged with murdering officer Sean Gannon had “earned the death penalty” and went after the state’s criminal justice system for failing the Cape Cod community.The police department said in the Facebook post yesterday the 29-year-old accused of murdering Gannon, 32, would forever be known in their building as “125” — the number of charges police said were on Thomas Latanowich’s probation record.