Roman culture was laced with strong ideals for honor and noble character. However individuals acted privately, the honorable identity was ingrained by society. The idea was so pervasive that it was considered a good mark even for an otherwise dishonorable person to commit suicide as a last act of nobility or to escape further dishonor. A noble condemned to die would generally be offered the opportunity to end their own life rather than face an ignoble execution. Death of an individual could be less an issue than the death of an individual’s honor. Being granted mercy [clementia] or a pardon was in theory a good thing, but on the flip side, it put the receiver of the mercy in the position of a submissive and was a humiliation. The person granting the mercy was also perceived as having greater power and as being virtuous.

Caesar had powerful connections and blood within his own family, but he wasn’t destined to be high ranking on birth alone. He worked long and hard to rise in his position and was eventually made governor of Gaul and Illyricum. The position kept him immune to prosecution and during his campaigns and governorship he grew in power and popularity, particularly with the people and his own soldiers. The Senate didn’t like where that was going and most likely planned to catch Caesar out on perceived crimes when his immunity expired. That scenario was part of Caesar’s motivation to initiate a civil war.

Caesar curried favor with the people and spent more time impressing the masses than the Roman elite. He was careful not to declare himself a king or appear as a tyrant, and instead wanted exude both power and benevolence. Caesar was also a man of honor, who didn’t seem to see a benefit in useless bloodshed or killing just for spite. When Caesar granted mercy he put himself in a position of greater power than those he granted mercy to, which also slighted those who were made to be supplicants to his mercy. Whether or not the Roman people wanted to see any members of those who’d been against Caesar dead, the imagery of Caesar being benevolent and peaceful made for a much better PR move than a lot of executions and separated Caesar from the image of a tyrant. While it irritated his own allies and didn’t endear himself to the people he was pardoning, it also didn’t give his enemies more fuel with which to decry him or move against him. Even if Caesar had been ‘justified’ in executing people who’d been against him, it would have been very easy to malign him as a vicious, bloodthirsty tyrant if the relative first thing he did after ‘taking over’ slash ‘not being king’ was kill a bunch of Romans. We don’t know for sure how much of his mercy was motivated by sincerity, calculation, or malice.

Caesar is often ‘quoted’ or written about through others [such as Suetonius and Plutarch] and Shakespearian quotes are mistakenly attributed as direct quotes. There are references to letters Caesar wrote, or had supposedly written, scattered throughout historical accounts. Caesar wrote [or dictated] at least two books called the Commentarii de Bello Gallico, and the Commentarii de Bello Civili that were his own accounts written in a third-person narrative. They have been translated, and differences in translations can create variations in phrasing and subtle meanings. There is a level of self-propaganda in the writing, but it is mostly intended to be a factual retelling of events rather than dipping into prose or metaphors. So he isn’t spilling the mechanics of his thoughts and motivations specifically. There are aspects of actions that he takes that are inherently sensible in his time that don’t need explanation. The culture of Rome at that time was quite different from a modern period in say a European country, so the context under which he makes his decisions isn’t obvious presently. But at the time the concept of the sort of superior and lesser mercy, the hierarchy of Roman people, the fears of tyrants and the social concept that placed high value on virtue and honor was intrinsic to the Roman way of life.

Caesar is quoted as having said, “He conquers twice, who shows mercy to the conquered”. But I can find no source for where he’s meant to have said that.

Caesar did in general show a policy of mercy outside of his actions in the civil war. For example, he hosted a triumph and displayed the captured Arsinoe, sister of Cleopatra VII, but then chose to grant her mercy by sending her to a temple rather than executing her as Cleopatra had preferred. There is speculation that Caesar had intended to pardon Pompey as well. Although we don’t know with certainty exactly why he made certain decisions, historians can make reasonable speculation and debate based on the context of the time and his actions overall.

Personally, I don't think it was in Caesar's character to kill without cause or let personal feelings drive him to make violent decisions. I don't believe he saw it as necessary, or he would have done it. If he had executed groups of nobles, and then especially if he had tried to take their property after, it would have reflected on him poorly and conjured an image he didn't want to reflect. Caesar did value honor and his country, and diminishing Rome by killing 'her people' was a disservice. I believe it very possible he took pleasure in turning the tables on people who went against him, but I don't consider that malicious or that he would make a decision so impactful based on that feeling. Mercy was a power move, a PR move, and in line with Caesar's values.