Those are just terrorists, like there have been before in Europe

Because ISIS is more of a terrorist organization than a military organization?

Why shoot up a public concert hall if your enemy is Putin and not the general public.

Why do any extremists - or russia, the US or israel for that matter - target violence towards civilians?

Maybe they believe the cause is worth it, the tactical calculus still comes out in their favour, or they just hate all russians? General destabilization? Forcing the state to devote more resources to protecting soft civilian infrastructure. Making people feel unsafe. Inspiring similar atrocities. The logic of tactically deploying murder isn't always clear to an outsider, especially before we have a firm idea who did this.

I strongly doubt this is an "honest-to-goodness sign of revolution". Shooting concert-goers is obviously not that. But I don't think it's logically sound to rule out an anti-putin motivation just because civilians were killed.

Because a couple dudes with small arms can't be effective by attacking the Russian military or police forces head-on. Instead they choose to attack soft targets and maximize the social disruption and impact by terrorizing citizens.

I would normally say it seems like a convenient false flag if they could pin it on Ukraine or something, but it sounds like several sources have confirmed ISIS is claiming responsibility.