The North was very responsible for slavery.
If you want to go in depth, read the book "Complicity" that was written by reporters for the Hartford Courant.
I don't know how deep you want to go.
Consider a few things
1) The North had both full and gradual emancipation
2) 1st Industrial Revolution in the US centered on textiles--cloth
3) Slaves' clothing and farming tools
3) North and international trade of American goods
4) Northern banks and Southern planters' relationship
5) Northern insurance companies' relationship with Southern plantation owners