If he had been successfulit could have caused "devastating effects on our city", he said.
When asked about additional suspects, Brown said he is still not convinced that Johnson was working along.
Discussing Johnson's journals being investigated, Brown remarked that he "obviously had some delusion".
"When all attempts to negotiate with the suspect, Micah Johnson, failed under the exchange of gunfire, the department utilized the mechanical tactical robot, as a last resort, to deliver an explosion device to save the lives of officers and citizens", the police statement said.
The details made for significant new disclosures about the killings that have rocked the Dallas community and the nation.
Police rigged a bomb-disposal robot to kill the armed suspect during negotiations Thursday.
Protesters are lining the sidewalks around the building, holding signs reading, "Does your badge still stand for justice?" and "Black lives matter more than white feelings".
"And it's sort of like being in a relationship where you love that person, but that person can't express or show you love back", he said. "That should be fairly easy". He said nine officers, including two El Centro police officers, were injured.
Some departments ordered officers to pair up or more generally said they were heightening security.
Johnson had told authorities he was upset about the fatal police shootings of two black men earlier in the week and wanted to exterminate police, "especially white officers", the Associated Press reported. Brown said he became increasingly concerned that "at a split second, he would charge us and take out many more before we would kill him". He says "while it's been established that hate speech is protected by the First Amendment, we're talking about people specifically saying on Facebook they want to kill white police officers".
Mr Brown said he stood by his decision, but understood why questions have been raised about the use of deadly force against the gunman, rather than opting for a non-lethal method to disable him.
"For our officers, they were suspects", he said. "And that's still in me, that keeps me going", Chief Brown said.
The wider crime scene includes the parking garage where Johnson was killed and at least two other sites where he is believed to have fired at officers.
According to the Dallas Morning News, the sight of a masked man put officers on high alert, just days after a sniper opened fire on officers guiding a protest march, killing five officers and wounding seven others. So far police are unable to understand the acronym's meaning.