Well, "MAGA people" were not doing any such thing.
MAGA assumed from the outset that he must be far-left, both because it confirmed their own biases and because they didn't want to believe otherwise. They did the same thing with the kid who shot Trump, though he turned out not to really have any political intentions.
What they were doing was mocking the occasional far-left dolt who tried to make the claim that the shooter was from the Right.
It's actually pretty plausible. The alt-right has been pissed at Charlie Kirk for years, especially Nick Fuentes' groypers, who have considered Kirk a race traitor. And there is some evidence that Robinson had sympathy for groypers, though at some point he decided he was gay and probably started to get pissed about Kirk's anti-LGBTQ screeds. Perhaps he shifted left generally, perhaps he remained generally right-leaning except on those issues, we don't know. The only information we have is a vague claim by his family that he had become more political and that he disagreed with them.
And yeah, we do know for sure that the shooter was a far-left nutjob.
We really don't know that for sure. On balance I think he probably had shifted pretty left, but the evidence is ambiguous at best. Maybe we'll learn more, but it's possible we'll never know unless Robinson decides to tell us.
If I had to put money on it, I'd bet that Robinson's politics were pretty muddled and his main reason for hating Kirk was the LGBTQ stuff.