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
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."