The older guy was probably disillusioned and felt disenfranchised. The entire thing however is irrational. I guess they were inspired by Wahhabi ideology that they seem to sell to Muslim youth all around the world to recruit people. When you look at tamerlane tsarnaevs youtube account you can find plenty of videos of Wahhabi speakers aswell as videos critical of kadyrov and the local government of chechenya. It doesn't make sense however since America's role, at-least of what has been revealed publicly is neutral in the conflict between russian authorities and Chechen separatists. At one point America even sheltered Chechen separatists who obviously belonged to armed groups in Chechenya. I guess their motive however was inspired by the Wahhabi's propaganda machine that disseminates Anti-American propaganda all over the globe, because America competes with Wahabis over the control in the middle-east. It's just the stupid kid, Tamerlane tsarnaev thought that it somehow would have an effect on anything.
I don't believe however, like others in the thread that it was narcissistic. It was more blind conviction to an ideology and disillusionment and the disenfranchisement that oppressed people feel when they're displaced. I'm sure Tamerlane felt angry and disillusioned because of all the bigotry disseminated by the strong anti-Islam movement in the USA and all the bigotry from the christian right. I can't imagine feeling emotional well-being when your culture and ethnicity are rendered powerless and you live in a society where ideological forces feed the population with hate against you. And that is exploited by people who give you a cause, no matter how irrational, baseless or stupid it is.