Hardline is an offshoot of straight edge, started by Al in boston in the late 80's. It was an even more intense version of straight edge. They were basically edge guys that would invade other's personal freedoms if they didn't agree with them. Kind of like hate edge.
"If Ian McKaye saw a guy drinking at an all ages show, he'd shoot him a dirty glance. If Al and his crew saw a guy drinking at an all ages show, they'd kick the shit out of you." -BBD vol. 2