Answer:
Cinna was killed for showing approval of the Revolution towards the capital in his works of art in Katniss's clothes.
Step-by-step explanation:
He was hurt in front of Katniss when she was going up the tubes that leads to the arena. He was later took to another room when there he was tortured until he told the capitol government the plans about the revolution , which he did not tell them about anything. The guards finally asked him, "Tell us about the plans for the revolution o you die", to which he later responded, "Never'. Then the guard shot him in the temple of his head and he died immeadiantly.