The 43-year-old Lee also had an obsession with the Discovery Channel, viewing the television company as an agent of environmental harm, The Washington Post reported.
Lee, who was shot as he held hostages Wednesday, was arrested in 2008 for throwing money in a protest outside the cable channel's headquarters in Silver Spring, Md.
"We are running out of time to save this planet and the Discovery Channel is a big part of the problem," Lee wrote in an ad he bought the same year in a free newspaper owned by the Post. "Instead of showing successful solutions, their broadcast programs seem to be doing the opposite."
Lee entered the Discovery Channel's headquarters building just before 2 p.m. EDT with a handgun, and with what he said was a bomb taped to his chest, and took three employees hostage, police said.
Three hours later, he was shot and killed because police said he was becoming more agitated and had pointed the gun at one of his captives. The hostages were not injured.
In writings posted on the Internet, Lee talked of the importance of saving animals.
"The planet does not need humans," he said.
CBS News said Lee wrote on the Save the Planet Web site: "Civilization must be exposed for the filth it is. That, and all its disgusting religious-cultural roots and greed. Broadcast this message until the pollution in the planet is reversed and the human population goes down! This is your obligation."