Complex sentence: In effect, about 2,000 people lost their lives in the Jaworzno camp; some of them were murdered by German civilian employees of the coal mine (mostly members of the paramilitary organization SA), who had been tasked with overseeing the prisoners at work.
Split into simpler sentences: In effect, about 2,000 people lost their lives in the Jaworzno camp. Some of them were murdered not by the guards but by German civilian employees of the coal mine (mostly members of the paramilitary organization SA), who had been tasked with overseeing the prisoners at work.