[Itse]

While I get why Russians would blame Ukraine, everything about this attack is just so ISIS that I think the story is more likely to be that Ukraine and the West are working with ISIS, so it's "all the same, really".

Of course, there is evidence that the US is still working with Islamic extremist groups in the Middle East (every major power likely works with some of them) so this doesn't even take THAT much spin doctoring.

However, regardless of how they spin this, this attack is still really bad news for Putin.

Putins claim to power has long been that "I get all the power, and in exchange you get a stable and prosperous country". (Classic stuff.)

If his security forces are shown to be weak and ineffective when it comes to protecting Russians, that really undermines his right to rule. It's especially bad that this was Moscow, he's so far been making an effort to protect Moscow and St. Petersburg from the worst of the war.

It's not just that this attack was apparently a success with the perpetrators getting away, this makes the security forces look weak, and everyone who doesn't like Putins regime will look at this and go "maybe we should try something too".
Russian security forces are already spread thin trying to protect a large number of targets against Ukraine. If ISIS enters the scene, they add a completely new and different set of targets to the list of things that need protecting.

I don't think Russians are much different than anyone else when it comes to demanding that their leadership do something when something awful like this happens, and even if Putin retaliates against Ukraine, that doesn't actually catch the people who did this attack or stop further attacks.

...also, eff ISIS.