One reason why Kremlin doesn't hide all the murders , is to send a message for anyone in the future, mafia style.So this is something that has always intrigued me about this avenue of "geopolitics".
The guy has already released all the evidence, other than basically confirming its true and they didn't want it out, what does killing him actually achieve?
You need to think of Putin not as a president , but as the leader of a Mafia organization , with all the rules coming with it.