Santos died from his injuries at the hospital.
Six children were confirmed dead, all aged four, as well as a teacher who was allegedly locked in a room and set on fire.
The municipality of Janauba announced a 7-day mourning in region.
A total of about fifty people were hospitalized due to injuries, 10 remained hospitalized in life-threatening conditions, said Bruno Ataide Santos, director of the hospital.
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A firefighter in the city of Janauba said by telephone that the guard died hours later in a local hospital.
Investigators are still trying to determine a motive for the brutal attack.
But police superintendent Renato Nunes told the website of the Hoje em Dia newspaper that the guard had mental health problems since 2014.
The guard had worked for at least eight years at the nursery school where he was not directly in contact with the children.
Officers found numerous bottles of alcohol at his home, Nunes said. Evacuation would have been more hard. "I want to express my solidarity with families", wrote Brazilian president Michel Temer on Twitter.
Temer said he understood "this must be an extremely painful loss" for the parents, as he, too, is the father of a school-age child.