Based on the information we know now, here is what I theorize will most likely be the defense argument at trial:
After suffering from years of emotional and physical abuse at the hands of his older brother Tamerlan Tsarnaev (aka Black Hat Guy aka Suspect #1), Dzhokhar was essentially brainwashed by his older brother and, just like Patty Hearst, he was suffering from Stockholm Syndrome. Although he tried to break away from his brother's abuse by attending college and living in the dorm; Tamerlan would not relent in his abuse of his younger brother and after Tamerlan's 6-month trip overseas in 2012 his manipulative abuse escalated and when he was around his older brother Dzhokhar was under his spell. Dzhokhar had to hide this abuse from everyone out of fear of his brother killing him.
It was after seeing the consequences of the devices built by his brother that Dzhokhar began to understand what he did. Dzhokhar tried to live a normal life desperately hoping that the whole thing would just go away; he attended class, slept in his dorm room and went to parties. After LE released the photos of he and his brother to the public Tamerlan lost it: Tamerlan raced to find Dzhokhar on campus and convince him to escape the area with him. After leaving the UMass-Dartmouth campus Tamerlan said he needed to make one stop at MIT, after arriving on campus a MIT officer recognized Tamerlan who proceeded to shoot and kill him. Dzhokhar got behind the wheel of his brother's Mercedes and tried to escape but Tamerlan was too fast and held his brother at gunpoint and forced him to drive.
A police chase began and the route went to Watertown. Tamerlan began to shoot at police and when he saw an opportunity to gain the upper hand he ordered Dzhokhar to stop. Tamerlan prepared his poorly constructed explosive devices and attempted to use them at approaching LE. After Tamerlan through the first device, Dzhokhar began to speed away. Conflicted he turned back and when he saw the opportunity to end his years of suffering he used his brother's Mercedes to kill his brother.
After killing his brother and ending years of abuse, Dzhokhar was terrified and wounded and believed police were going to kill him. Dzhokhar needed to find a place to hide and wound up choosing a boat behind a house. After killing his brother, Dzhokhar did not commit any further acts of violence or terror. He did the right thing by surrendering to LE. He clearly belongs in an institution where his mental health can be repaired from the years of abuse he suffered at the hands of his brother. He deserves help, not prison; he wants to pay for his actions but we owe this victim of unimaginable abuse compassion.
This is not at all what I feel should happen, it is simply the most likely and most effective defense that he could present given all of the information currently available.