You are right, the Soviet KGB would not have been that sloppy. However, I am not sure that they would have killed a man like Denis Voronenkov, but now Putin has replaced KGB by other organs.
The killer is Pavlo Parshov, “an agent of Russian special services who infiltrated Ukrainian defense agencies.”
Crime Russia is a Russian site for “active measures” https://www.theguardian.com/world/2016/may/07/discovered-our-parents-were-russian-spies-tim-alex-foley