Eugene H. Spafford, a computer security professor at Purdue University, was not convinced that the arrests last week would serve as a deterrent. Rather, he said, it could prompt others to be more careful in the future and even prompt retaliatory strikes. “A whole bunch of people were angry, they didn’t really think about whether it was legal or not. It never entered their minds,” Professor Spafford said. “This was kind of the equivalent of a spontaneous street protest, where they may have been throwing rocks through windows but never thought that was against the law or hurting anybody.”