Putin almost single-handedly ended democracy in Russia and has been running the entire country with an iron fist for over two decades. He's only a "moderate" in the sense that he hasn't tried to bring Communism back.
Putin's #2 is a boring ass bureaucrat with no meaningful controversies that I'm aware of except typical Russian corruption. There are other scary people in Putin's cabinet who might try to take power if Putin dies (Igor Sechin), but at worse they'll be similar to Putin rather than anything worse than him, and probably smart enough to back off considering Putin appears to have gone off the deep end to the detriment of the country.
Not to say I think anyone should try to assassinate Putin. That will only heighten and prolong conflict between Russia and the West and potentially strengthen people's resolve. Nothing is going to change in Russia unless it happens from within.