You were banned for saying "Kill billionaires".
Not for saying "Luigi Mangione isn't guilty tho".
Even if you didn't mean it as an invitation or incitement to homicide and merely as an answer, completion, or riff to the previous statement, on its own it can be interpreted that way.
I'm rooting for Luigi too, I don't feel sorry about the death of the UnitedHealthcare CEO, and I do think billionaires shouldn't get comfortable with profiting from our suffering. But the mods reserve the right to remove stuff they feel could get them in trouble, chances are someone did interpret it this way and made a report (wasn't me I swear). There may be certain contexts, communities, instances, where this is allowed but you will have to determine that or ask.