President Snow had him executed because, in Snow's view, Seneca failed to adequately handle the situation with Katniss, Peeta, and the nightlock.
The Question and Answer section for Catching Fire is a great resource to ask questions, find answers, and discuss the novel.
President Snow had him executed because, in Snow's view, Seneca failed to adequately handle the situation with Katniss, Peeta, and the nightlock.
From the text:
the town square has been transformed. There are machine guns along the rooftops, and whipping posts, stockades, and gallows in the center of the square. The Hob has been set on fire and is reduced to just a pile of rubble and ash.
I think in Catching Fire, they do agree on allies.