Who Killed Sara? isn't saying outright that César killed Sara, but they've certainly implied it. César's the one that talked Alex into taking the fall for the Lazcano family when Sara was first killed and it is César's baby Sara was pregnant with at the time of her death. The only person César looks out for is himself. That's clear from how he treats his family and the women he sleeps with.
We already know that César feels he's above the law, due to the criminal comings and goings at his casino and the brutal murders that happen there at the hands of his colleague Sergio. He also knew that Sara was pregnant with his child and likely knew more than she let on about the sinister nature of his casino. In one of the final flashbacks of episode 10, we see César shoot someone but it's unclear if it's Sara. We also know in the present day that a skeleton with a bullet hole is uncovered in Alex's yard. Could it be Sara? He does tell others he's not a killer but he's also a liar so what are we to believe.