The greasegun is a super cheap .45 cal submachine gun developed by the US to arm revolutionary and resistance fighters with who's actions we thought would advance our interests. We did not radicalize those fighters, that had already been accomplished by the occupying powers and the illegitimate regimes established by those powers to support their agendas.

Point being, who gave this guy a weapon is interesting, but it doesn't have much to do with why he used it.