That’s true — it was a kneecapping move. Here’s where we could fault the North for not banning it sooner. But embroiled in a war, the North would have found that more difficult than waiting till after the war.
If the North had wanted to ban slavery they could have done it at the 1787 Constitutional Convention. But at the time, it was not in the North's economic or political best self-interest.
The North could have initiated the peaceful abolition of slavery through a constitutional amendment at anytime prior to 1860. They did not. It was not in their economic and political best self-interest.
After 300,000 southerners had been safely buried the North passed a series of constitutional amendments. It was in their economic and political best self-interest.