Hasn’t Islam been gaining converts by the sword since the beginning?
Further, even if we could point to some who took up Mohamadism voluntarily, how many of those did so in order to later take up swords to conquer and "convert" their enemies?
In this particular example, the victims were European consulate employees, families and merchants, so "conversion" was clearly not the issue.
But the massacres also included lootings, so perhaps there was some economic motive mixed with their religious fervor?