I don’t know. Just easier and cheaper than buying one. Why would he have kept the pistol and suppressor on him instead of disposing of it? Why have the manifesto addressed to the feds on him? Normal people aren’t cut out for living with the guilt of murder/fear of being a fugitive. On the one hand, Luigi obviously put a lot of planning into the adventure, but seemingly very little into the “getting away with it” part. Also, no self-inflicted gunshot, no suicide by cop shootout.