As for a corrupt republic, this is how I see it. The late republic was a corrupt shit show controlled by self serving patrician families who would take massive loans to bribe their way into office, complete their term, then go out and govern a province to loot and pillage even more money in order to bribe their way out of prosecution for their crimes when they returned to Rome. Meanwhile the vast majority of them had little interest in reforming any of this or the multitude of other issues plaguing the city. Those that did take an interest in reform(wether motivated by genuine concern, lust for power and prestige, or a mixture of both) Were opposed and often killed by the rest of the patrician families. This was the status quo, and it was not tenable. Compare that to a regime which was actually effective at reform and far more concerned in the interestes of classes besides the patricians, and I think it's an easy choice. Calling the former a republic does little to change my mind on that.
Caesars campaigns in Gaul were not exceptionally brutal compared to other Roman campaigns even if they were exceptional in terms of scale(though Pompey's Eastern conquests were similar in scale). In fact, though Caesar was capable of brutality when he felt it was needed(the story of having all the hands of warriors in one tribe cut off after they broke a peace treaty and went to war with him again comes to mind)I think his rather lenient treatment of guals after the defeat of vercagetorix and his behavior compared to the optimates in the civil war( who were far more brutal as a policy), and his insistence on forgiving enemies(compare this to Sulla) lends credence to the idea that he was not some sadist who enjoyed inflicting pain on others, and was in fact inclined to forgive.
I'm almost done ranting, I just suggest you keep in mind that Caesar didn't even plan on becoming dictator for life either. He wanted to be able to run as consul and then gain another province in order to keep imperium and avoid being destroyed by his enemies in court(which he was legally allowed to do).He was pushed into a corner by Pompey and the optimates, which amounted to unconditional surrender, and chose to march after several attempts at coming to a compromise failed.