。
Slaughtering prisoners of war is a cruel act.
Literal
Prisoners-of-war [obj-を] massacred [nominalizer-のは] cruel act is.
A cleft sentence: ~のは~だ highlights the subject (the slaughtering) as the topic and predicates 残忍な行為 of it. 捕虜 ('prisoner of war') and 虐殺 ('massacre') are both Sino-Japanese compounds with heavy semantic weight; 虐殺 specifically implies atrocity, not just killing. 行為 ('act, deed') is a more formal, moralized word than 行動 ('action, behavior').