There are more people that Cassian has killed for the Resistance. Never such a blatant assassination, done to make a point.

"It was my idea," he says, bluntly, suddenly humorless. "What else would you call me?" The question is some combination of rhetorical and serious; he doesn't believe there's any real answer that Poe can offer, but he would listen if Poe did offer one.

His idea, his mission. His careful study of the planet's political situation, though he was backed up by other operatives, analysts. If this goes sideways, it's his fault.