First thought that came to mind as well, it sounds very implausible for him to be carrying a manifesto.

Ikr

Well, a YouTube account with the shooters name posted a video today titled "truth" which was basically saying that there were videos scheduled to be made public in the near future (presumably on the same account), which would speak to his motivations and mind set or something of the sort.

Instead of telling you about it, I would prefer to just link the video, but according to YouTube, the account associated with the YouTube channel has been removed, so you can believe me or not on this. the video was only up for about 4 hours and when I saw it, maybe 30 minutes before it was removed from YouTube, it had 10's of thousands of views. I know those counters aren't exactly real time, so it's not unreasonable to think that upwards of 100k people saw it, maybe more, and unless one of them saved it, it's likely that nobody else will see it.

This thing about a guy at Macdonald's recognizing his eyes also sounds bit strange tbh

Or he consider himself a crusader for a cause more than a vigilante and was carrying it to get more attention to his beliefs.

My tin foil hat theory is that he was planning out another shooting and got interrupted.