I may have missed something, but my interpretation of that quest was that the contact betrayed her by going to the noble, then the noble double-crossed (triple-crossed?) him once he was no longer needed. And then her perspective on that was "That guy was a crap friend, but he's dead now, so it's all good."
That might be an irresponsible outlook in the long run, but that's the point. She doesn't care about the long run. But she still didn't make any effort to kill the contact. As far as I can see, she didn't even put him in a situation where she should have expected him to get himself killed. You can't seriously expect anyone to include looking after traitors, and Sera rarely plans more elaborately than "throw bees at the problem".