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.

Hard to say. It's not like the movies. With the firearm he had (.308 with ~14-16" barrel), a silencer/suppressor only brings the volume down from "instant tinnitus" to "still loud enough to cause permanent hearing damage". It's far from silent. I'd never thought about it until now but I wonder if a silencer/suppressor could have increased surviveability. When people heard gunshots at volumes they would expect, they might be less disoriented. Verses the muzzle flash and concussion that comes from what he was shooting (basically a semi-auto flash bang).

Probably had nothing to do with the tumor

BRUH, this was his suicide note:

I do not quite understand what it is that compels me to type this letter. Perhaps it is to leave some vague reason for the actions I have recently performed. I do not really understand myself these days. I am supposed to be an average reasonable and intelligent young man. However, lately (I cannot recall when it started) I have been a victim of many unusual and irrational thoughts. These thoughts constantly recur, and it requires a tremendous mental effort to concentrate on useful and progressive tasks.[43]

In his note, Whitman went on to request an autopsy be performed on his remains after he was dead to determine if there had been a biological cause for his actions.

People in the 60's didnt just say "do an autopsy on me" unless something was severely wrong. There was little to no public understanding of neurology, the general public wouldn't even think to guess that a brain tumor could play such a role.

And not like Whitman suspected it a little bit; before the incident he went to many doctors for help. This was his note in his journal

"I talked with a Doctor once for about two hours and tried to convey to him my fears that I felt overwhelming violent impulses. After one visit, I never saw the Doctor again, and since then have been fighting my mental turmoil alone, and seemingly to no avail."

He talked to friends about it and nobody would take him seriously because they just saw him as a respectable person with overblown concerns. His case is part of Neurology classes in Texas universities!