Its pretty easy to believe Putin is into knocking off his enemies, however this statement is not proof. Mere speculation.
The body count of prominent Putin critics continues mounting, so it's not as though the current Kremlin boss is about to clear up the speculation he is responsible, but the direct opposite, by remaining mute on the subject, as other critics are murdered.
Proof comes in all levels. It is in real life almost never like a mathematical proof, but instead is a preponderance of evidence. Remember that "proof" means test, not an absolute certainty.
Can anyone really belief that Putin is not behind this? This is the KGB-man whom we are constantly acting to help us out, and he always does his best to encourage our enemies and to pull the rug out from under any substantive controls on rogue states like Iran and North Korea.
You cannot trust Putin. You can, however, rely on him as an enemy: that is a sure thing.