Perhaps the guy was try to commit suicide by cop, not realizing the Rangers werent' armed (the backup BPD evidentally was).
“We were sitting under the tree watching the duck and then suddenly there was a police officer drawing his gun and there was a man real close to him, like 3 feet and he kept saying ‘shoot me, shoot me,'” one witness said. “The officer didn't shoot him, they finally got him on the ground, cuffed him and took him away.”
http://www.whdh.com/story/26785616/park-ranger-stabbed-in-the-area-of-boston-common
“We were sitting under the tree watching the duck and then suddenly there was a police officer drawing his gun and there was a man real close to him, like 3 feet and he kept saying ‘shoot me, shoot me,'” one witness said. “The officer didn't shoot him, they finally got him on the ground, cuffed him and took him away.”
http://www.whdh.com/story/26785616/park-ranger-stabbed-in-the-area-of-boston-common