why did the northerners believe slavery should be illegal?

Question

i need more reasons than they thought was immoral.

Answer 1

Some felt there wasn't any economical need for it as technology i. e. the Cotton Gin had been invented.

Answer 2

It stopped progress. It had become culturized and therefore against what the American spirit of individual achievement was all about. In other words it was un American. There was a cotton gin that could out do any number of slaves. It was a hindrance on progressiveness, that urge to move forward that is embedded in the white race.