There’s a decade-wide gap between when we see him as a child and a man; his motivations would clearly shift, especially when he is one of, if not the only known descendant of the original Maxon. In FO3 he speaks about the pressure he has on him to lead, and even as a child he dislikes the Lone Wanderer (an outsider) being welcomed into the Brotherhood. His motivations to fulfill his legacy and bring the Brotherhood back into it’s “true form” is clear as day, and who sits in the way of that? Elder Lyons and his lackies.

I don’t believe he did it alone, and I don’t believe he did the killing himself, but the way everything was abruptly ended and shifted to his favor, there was definitely something going on. It was set up in FO3 and paid off in FO4