Lee's troops behaved much better in the North than Northern troops in the South.
The fact that the destruction visited on the North was far less than that visited on the South is indeniable. Lee and his army spent very little time in the North. And in many cases, such as in Georgia and in the Valley, the destruction of goods and facilities that benefitted the confederate war effort was done to deny those resources to Lee's army. But the claim that the individual actions of the soldiers was 'much better' is debatable.
The iron mill was destroyed by the orders of General Early. That was a military target. It was only a side benefit that it was owned by Thaddeus Stevens.