Efficiency. Marcus didn't waste time. He was -always- hunting and he it seemed like he was always ready for a fight. I'm probably one of the most aggressive pkers. Marcus was more so, and better at it than I am by a mile.
He also found people. Always. He could look at the who pk and figure out who was grouped with who and where that group would most likely be ranking. His chars have even found me ranking in holes like Crystal Island where no one goes.
Underrated aspect of Marcus, much like Shaapa, he was an OG killer with or without good suits. Marcus inherently took risks and he -did- die. Those risks don't always pan out. But his recovery ability was phenomenal. You could full sac a Marcus char, he'd go grab some skull rings and charred bracers and literally PK his way into an elite set within 12-24 hours.