Seneca Crane was executed after Katniss's and Peeta's suicide stunt at the end of the Games, and the act was intended to remind the Gamemakers of his fate and suggest that they could suffer the same.

Why did Seneca Crane have to die?

Seneca is first mentioned by name when President Snow revealed to Katniss that Crane was executed for not killing Katniss and Peeta when he had the chance. During her individual session with the Gamemakers, Katniss is trying to find some way to display her defiance. ... President Snow didn't act like it was.

Why did Katniss show Seneca Crane on the dummy?

On the dummy, she writes Seneca Crane. Seneca Crane is the name of the last head Gamemaker. He was killed after allowing Peeta and Katniss to both win the previous Hunger Games. Katniss does this to remind them that they, even though head hanchos for the capitol, are not safe from President Snow's wrath.