Lost the finger at the massacre, which-- plot twist-- turns out to have taken place not THAT long ago (they never say exactly when it was in the episode). Star's mother wasn't the queen, but Miss Heinous or someone related to her might have been.
Somehow weak against magic, moreso than other monsters (this theory's been explained here before: him losing that arm might not have been Star's intention when attacking him; it looked like the same spell she used on Ludo earlier), maybe as a direct result of whatever happened at the massacre. Destroying the wand may tie into that, whether to weaken the Butterfly family or somehow benefit himself through the cleaving or the consequences of the explosion.
Or he may have temporarily had a higher position within society, maybe even working in the castle, but something happened that changed all that.
Most general plan is probably just some convoluted revenge thing.
For as weird as the finale was no one, including me, seems to have gone too out there with theories about him yet, maybe because he's just not in it that much.>>77497165
I'm not crazy about this either, but next to "future Marco" almost anything seems tolerable.