In Roman times, gladiators were big money makers. A lannister was a guy who basically was a pimp for the gladiators. Different from a trainer of gladiators. He would usually be spotted in the prisoner camps after a war, picking out the men capable of handling a fight and sending off the rest to their vocations. An HR executive or a talent agency in modern parlance.
Pompous, online, gamer lacking in both talent and modesty. Usually found whining most of the time rather than working on improving skills. Hated by all.
He started his nasty reputation by lying to Sansa, then going on to execute the Lord of Winterfell and Warden of the North Eddard Stark after he confronts Queen Cersei and tells her that his son
shouldn't be king anymore.
Long after the death of Lord Stark, Joffrey decides to torture Sansa and get her to strip in front of everyone, including his family. After she demanded to "stop" talking about her family. Luckily, Tyrion came and rescued her.
Joffrey soon fell in love with Margaery Tyrell betraying Sansa Stark. On his wedding day with Margaery, the tables turned as one of the most richest man in Kings Landing is poisoned by Olenna Tyrell (Margaery's Grandmother) and Petyr Baelish (Littlefinger). On his last seconds, he points at his Uncle Tyrion for the murder and gets him arrested.