For the longest time, "Who Killed Sara?" certainly implies that César is the one that kills Sara. César's the one that talked Álex 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.
During season one, César tells the others he's not a killer, and by seasons two and three, we see his statement holds more truth than expected. While César has killed in the past, including the body buried in Álex's backyard, he doesn't do it just for fun, unlike his associate Sergio. He kills Abel Martinez, Sara's real father, because he tries to murder Sara. Although César may be a shady businessman and an even sh*ttier father, he's not the one who killed Sara.
