Yes god forbid you waste five seconds second in introspection.
I'm a mindless Putin slave, what do you want from me, some sudden realization that will turn me to the True Path Of Democracy™?
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But yeah, in regards to Putin not being an idiot and having plenty of ways to kill his opposition - true. But a quiet death of a big figure raises greater alarm and pretty much guarantees only someone with extensive resources did it. Such as Putin.
Gunned down in public? Yeah, could have been anyone. Also sends a very clear message if it was him. Dissidence equals lead.
Remember, this is the man behind Putin's polonium poisoning bonanza. He knows full and well the value of public executions when it comes to silencing dissidents.
By that logic, any death of a big figure could be attributed by Putin.
Also, last time these dissidents had any political power (2011-2012 mass protests), there was a conspicuous lack of murders, even though by that logic, he should've gunned at least some of them down to suppress unrest. Instead, he just let people to roam a little, shouting obscenities aimed at him, to blow off their steam and return to normal life. A few people got arrested, but that happens at every major protest movement, like Occupy Wallstreet.
If he didn't do that then, why should he suddenly start to do so now, against a completely powerless (10 000 people on the march is the maximum extent of their capabilities) opposition that do not have any ability to hurt him, and whose deaths would only bolster and galvanize the opposition movement by creating another martyr?