Well, Alexander Litvinenko for one. He also attempted to assassinate Viktor Yushchenko, but he survived.
Then there are all of his critics who tend to get mysteriously gunned down around Moscow, like Victor Nemsov, Anna Politkovskaya, Paul Klebnikov, Sergei Yushenkov, and Stanislav Markelov.
Litvinenko and Politkovskaya are the two cases always put forth as evidence, yet they're the worst. Politkovskaya was detained by Russian forces in a guerrilla war in Chechnya 5 years prior to her death. But we are to believe Putin waited for a more opportune time, like when she was living in Moscow, with all kinds of witnesses/associates around, and then hired Chechen organized criminals to gun her down near her apartment. And Putin was then able to manipulate Belgian police who uncovered this crime and the killers??
For any serious person looking into the cases you cited, they will find them even more ludicrous than the MSM anti-Trump propaganda. Look, I understand the neocon reason trying to undermine Russia, but just don't expect intelligent and serious people to buy it. As a truth at least.