The Tsarnaev brothers acts are indefensible, in part because of what they did--targeting innocent civilians for death and injury. But, they are indefensible for other reasons as well:
a) the brothers apparently made no attempt to publicly justify their actions. By contrast, Osama bin Laden spoke on many occasions about the link between 911 and America's role in the Mideast. It is true the news media refused to print transcripts of his messages and substituted the nonsensical and self-serving theory that bin Laden hated us for our freedoms, but at least he made an attempt and made concrete, if abstract, offers to stop terrorist acts in exchange for changes in US policy. He may have committed evil acts, but they were not nihilistic by nature.
The Tsarnaevs, by contrast, engaged in what used to be called propaganda by deed, but left out the propaganda. Were they punishing the US for killing and maiming innocent civilians as collateral damage in various parts of the Muslim world where our military is fighting? Were they punishing the US because the FBI acted at the behest of the hated Russians, deniers of Chechnyan independence, when they interviewed Tamerlan in 2011? Or, was it to oppose the US and help bring about Islamic states in majority-Muslim countries.
Until they were identified one couldn't even be sure they weren't right-wing anti-tax and anti-federal government fanatics maiming liberal Bostonians.
b) If one is going to commit mass violence against civilians and there is no explanation there can't be a positive political consequence. It does not force some members of the government to re-think the costs of existing policies, or get the public to do the same in terms of their allegiance to those policies. The Tsarnaev's did not seem to have any idea of how their terrorism would advance their political goals, whatever they were. They made no demands, They did not offer a quid pro quo for ceasing future bombing. Their violence could not have any positive outcome unless reducing the population of living and able-bodied Americans by a handful was a goal in itself.
c) The actual consequence of their actions was to make life even more difficult for American Muslims and those abroad. On the domestic front, Dzokhar's Tsarnaev's affable and fully-Americanized lifestyle will make even the most assimilated Muslim youth and adult seem like a potential subversive to some. If a high school bully picks a fight with a Muslim and the victim's friends say "He's just like one of us. He's cool," the retort will be "So was the kid in Boston." Profiling will accelerate at every level of society.
Abroad, the bombing will give more leverage to those who want us to be even more aggressive in interfering in Muslim societies. Purely local insurgencies will be seen in terms of a new version of the domino theory.
I do believe there is some truth to the view that one person's terrorist is another's freedom fighter, depending on historical analysis and values. But, what was done in Boston should not even be part of that conversation.