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Marilyn Manson showed a shocking lack of empathy Sunday night as he pretended to spray the crowd with bullets from a fake semi-automatic weapon ... in a county that is still reeling from a mass shooting.

Marilyn --- in a wheelchair after a September injury that crushed his leg -- was performing at the Glen Helen Amphitheater in San Bernardino ... the city where a terrorist killed 14 people in December 2015.

Perhaps with that in mind -- and a penchant for controversial headlines -- Marilyn pulled out the fake rifle while performing "We Know Where You F****** Live."

A mic is attached to the scope of the rifle.