It must cause him uncertainty, because it undermines one of the fundamental rules of a mafia state.
Putin is not a democratically elected leader. He is a dictator whose rules are based very much on immunity that he provides for his enablers, for the Russian elite inside Russia. Also, after they commit crimes in Russia and steal money, they are able to keep this money in a safe harbour abroad.
Those are two very important elements of Putin's grip on power. He was seen as the only one who could guarantee such a comfortable scheme of getting rich and protecting their funds, and eventually even sending families to live comfortable lifestyles abroad.
Any meaningful act that will attack the interests of this group, the Russian elite, even at the mid-level, will have a dent. It could even be a massive dent in Putin's war chest, because the way the mafia operates is full loyalty for the boss in exchange for full protection. Every hit man must be protected.
I'm not saying that all these people committed really bad crimes, though in the case of Magnitsky we understand it was mind-boggling that Magnitsky was prosecuted posthumously for the crimes that he uncovered. That's why Putin and his cronies and his agents and his lobbyists were so aggressive in trying to repeal the Magnitsky Act. It is because it will hurt the very foundation of his so-called social contract with the Russian elite.
Would it lead immediately to his demise? I don't know, but I can tell you that the opposite is that if he sees any weakness, such as trying to repeal or even to soften the sanctions, that will be seen as his victory. The way Putin has been operating in the last few years, after his aggression in Ukraine and other aggressive foreign policy endeavours, has been to try to present himself to the Russian public, which lives under this 24/7 poisonous propaganda, as the only saviour of the country against global conspiracy. He knows that dictators can make many mistakes, except one: he cannot look weak. That's why he needs victories.
Unfortunately, now he is able to present many of his foreign policy acts, whether they were criminal—such as the annexation of Crimea or the war in eastern Ukraine, or other acts of meddling in elections and political life in Europe and America, or genocidal wars in Syria—as elements of his strength. That's why I keep repeating that anything that gets under the skin of a dictator, anything that makes him look weak, especially if it's a foreign-born defeat, is extremely valuable.