The issue is that this is not an isolated incident. Some people are stubborn, and will stay no matter what. Look at Fidel Castro, that guy survived plenty of attempts on his life, but he stayed. Now, the guy was a shitty human being, but he still took it all.
I'm not saying Putin's political opponents are all good people, some of them are evil bastards. However, the number of bodies continues to pile up. The number of people tossed in prisons... is stacking up. It's not as if Putin has hidden the fact that he doesn't really care about body counts. If he sees someone in his path, he will go after them.