Complex sentence: Many Jewish males of conscription age were already serving as slave labor for the Hungarian Army's Forced Labor Battalions where most of them died, including many who were outrightly murdered after the fighting had finished when they were returning home.
Split into simpler sentences: Many Jewish males of conscription age were already serving as slave labor for the Hungarian Army's Forced Labor Battalions. Most of them died, including many who were murdered outright after the end of the fighting as they were returning home.