Green Arrow isn't typically portrayed as an asshole. He's often a hero with a strong sense of justice, but like any character, he has his flaws and moments of difficult choices.
Green Arrow's back story involves him being a wealthy playboy named Oliver Queen who survives a shipwreck and develops his archery skills and a sense of justice on a deserted island.
Green Arrow's training in the comics was a complex process. It wasn't just one person or event. His own perseverance, along with learning from various challenges and encounters, shaped him into the skilled hero we know.