God judges sin, that doesn't make Him ruthless, it makes Him God. The book of Ruth reveals that the law which may appear ruthless is in fact judging your heart and not your outward appearance. Ruth was a Moabitess and according to the Law could not enter into the congregation for ten generations, however she was also the widow of an Israelite and so according to the law the kinsman redeemer was to marry her. How do you decide? Ruth decided. The story begins by her refusing to return to her people but instead saying "your God will become my God and your people will become my people".
The 3,000 killed for the worship of Moloch was not random, that was God judging the heart. Moses made everyone drink the bitter water. According to the law if you suspected your wife of adultery you brought her to the priest, he gave her the bitter water to drink and if she swelled up she was guilty. After giving them the bitter water these 3,000 swelled up revealing they were guilty and they were killed.
Jesus is our atoning sacrifice. You are a sinner, but if you receive the blood He shed for your salvation you will be forgiven. If you reject Jesus and His blood and the work He did to pay the price to redeem you then you will not be redeemed by the Lord, you will not be forgiven, and the wages of sin is death. This is not "ruthless", this is "righteous".