These pagan sites sometimes involved human sacrifice, but the Israelites would find themselves killed at these sites. They will see the utter futility of these idols they worshipped.
However, a remnant would be preserved. The remnant would realise what had gone wrong, how they had grieved God by worshipping idols and lusting after other things.Then they will be appalled at their behaviour.
People may say it is terrible that God can inflict such judgements, but the really terrible thing is that we live such lives that deserve such judgement, The nations around them would see the judgement and understand.