You don't know why someone is speeding, and it's not your job to enforce traffic laws. There was a story I remember reading a while back of someone's friend having a chainsaw accident, and them watching their friend bleed out in their car because some dumb piece of shit figured they'd play traffic cop and blocked them in traffic. If you see someone driving aggressively, just assume they're driving that way because they're about to shit their pants, and let them go on their merry way. Report them if you want, but don't endanger yourself and others trying to be a "hero"