That's a pretty big possibility, hardly would call it a presumption. Either way, humans are too frail and unpredictable to have limitless access to such killing machines.
Probably had nothing to do with a tumor:
Whitman's father was raised in an orphanage in Savannah, Georgia, and described himself as a self-made man. His wife, Margaret, was 17 years old at the time they wed. The marriage of Whitman's parents was marred by domestic violence; Whitman's father was an admitted authoritarian who provided for his family but demanded near perfection from all of them. He was known to be physically and emotionally abusive towards his wife and children.
This dude in Maine had tried to get a silencer. Wonder how much worse it would have been if everyone didn't hear the reports and start running.