One way to think about this is by looking at repeat offender criminals. Is there something about them, innately, that makes them commit crimes, or are there environmental factors, including their personal life histories, that contribute to their careers? Whatever topic one chooses, it is important to note that most social scientists are not reductionist--in other words nature vs. nurture is not necessarily an either-or proposition. As for Julius Caesar, you could argue, perhaps, that Caesar was born ambitious, and that he would have eventually accepted the crown and destroyed the Republic had he not been assassinated. Or you could argue that the political system in Rome contributed to creating ambitious men like Caesar and Antony.