The deranged mass shooter who killed two men in an Oregon supermarket appears to have written a disturbing online manifesto bragging about his plans to commit a “national tragedy” — initially planning a school slaughter “inspired” by Columbine.

Ethan Blair Miller, 20, was named by police Monday as the gunman who killed an 84-year-old customer as well as a heroic 66-year-old employee who tackled him as he sprayed shots through the Safeway in Bend late Sunday.