Steven Altman said the Indian-born Dharun Ravi was surprised to turn on his webcam and see his Rutgers University roommate in an intimate situation with another man. Altman emphasized that there was no recording, no broadcast and no YouTube video of the encounter.
The roommate, Tyler Clementi, committed suicide in September 2010, days after the alleged spying.
Ravi faces 15 criminal counts, including invasion of privacy and bias intimidation. He could be sentenced to 10 years in prison if convicted of the most serious of them.
Altman said Ravi was not acting out of hatred of his roommate or gays in general.
"If there's hate in Dharun's heart, if there's ugliness in Dharun's heart, where is information and evidence to support it?" he asked the jurors.
As the prosecution began its closing arguments later in the day, Julia McClure told jurors there's abundant proof that Ravi had a problem with Clementi being gay, telling friends his roommate was gay almost as soon as he learned who his roommate would be.
"There wasn't one thing about the defendant's actions that were good-natured," McClure said.
Jurors are expected to start deliberating by Wednesday.
Ravi did not testify, though jurors did see video of a statement he gave to police.
There's no dispute that Ravi saw a brief snippet of video streamed live from his webcam in the Rutgers dormitory room he shared with Clementi to the laptop of a friend in her dorm room on Sept. 19, 2010.
The friend, Molly Wei, said Clementi and his guest - identified in the trial only by the initials MB - were fully clothed and kissing at the time.
Ravi posted a Twitter message that night that concluded: "I saw him making out with a dude. Yay."
Two days after the first incident, Clementi asked for the room alone again.
This time, Ravi tweeted: "Yes, it's happening again" and "dared" followers to connect with his computer to video chat. There was testimony that he told one friend that there was going to be a "viewing party."
But there was no webcast. Ravi's lawyers say it's because he disabled his computer before Clementi had MB over.
In his summation, Altman noted that none of Ravi's roughly 150 Twitter followers seemed to take action after seeing his tweets. "Not one attempt to see anything, what does it tell you? Nobody cares, and nobody's taking it very seriously."
The challenge for jurors could be deciding whether the laws apply to what Ravi is alleged to have done.
One of the invasion-of-privacy charges accuses Ravi of viewing exposed private parts or sex acts - or a situation where someone might reasonably expect to see them.
Another accuses him of recording or disseminating the images to others. There's no evidence that the webstream was recorded, and witnesses said Ravi wasn't there when Wei opened the webstream for other students.
The bias intimidation charges could also be complicated. Ravi can be convicted of intimidation if he's also found guilty of an underlying invasion of privacy charge. Two of the four charges of that crime are second-degree crimes punishable by up to 10 years in prison.
Each of those charges says Ravi committed invasion of privacy - or attempted to - out of malice toward gays, or that Clementi believed he was targeted because of his sexuality.