In Season 1, Joffrey escalated from whiny twerp to merciless dictator when he ordered the death of Sansa's father, Ned Stark, after promising to spare his life. Slouched on the throne, Joffrey spends most of Season 2 making poor decisions just to get a rise out of those who surround him. He tormented Sansa because he could. He mounted a war because he could. He starved the people of King's Landing because he could.
When the world pushed back, his frothing mouth only spat bigger bubbles. When the truth of his lineage became clearer, the guy ordered the murder of every purported bastard child in King's Landing. Joffrey loved to flex his psychotic muscle, even when showered with gifts. When Tyrion presented the sexually frustrated teenager with Ros, Joffrey forced her to beat up another prostitute and eventually shot the working woman with a crossbow.