The sentiment in Washington before the Moscow attack was that Russians needed to experience firsthand the impact of the war in Ukraine. This attitude intensified as US sanctions proved ineffective, Russian advancements in Ukraine quickened, and President Putin received a high level of support from the Russian population during recent elections.
Previous attempts to make Russians understand the war's effects included drone strikes on major city centers, attacks on infrastructure, and efforts to destabilize Russia's economy.
https://www.csis.org/people/emily-harding
Would that really be the case though, or is it perhaps more that most people don't bother trying to apprehend them? When those guys back in France managed to kill Charlie Hebdo, they also tried to make a run for it. The guys who conducted the Boston marathon bombing (who I think I also recall hearing they swore allegiance to ISIS? Not sure though) also tried to make a getaway. I don't know anything about Russian police tactics, but it could be they're more likely to try and apprehend mass murderers alive when possible.
Besides which, if they survive they get to conduct more terror attacks. I don't think whoever's responsible for this attack counted on them being apprehended.
More often than not, they see themselves as martyrs, but you're right it doesn't conclusively prove they're not ISIS on its own.
wikipedia (i know, i know) has this to say:
"During questioning, Dzhokhar said that he and his brother were motivated by the wars in Iraq and Afghanistan, that they were self-radicalized and unconnected to any outside terrorist groups, and that he was following his brother's lead."