This was because he essentially did what he pleased with his four legions and didn't get permission for anything from senate.

This bit, not so much. Also, he wouldn't have been executed - execution was a punishment that essentially just didn't happen to upper-class Roman citizens at this point. Rather, they would be "sentenced to death," they would "flee execution," and settle down in a rather nice city - Massilia (Modern Marseilles) being one of the more popular destinations for this sort of thing. The only downside was the whole "being exiled from Rome" thing, which was pretty significant, especially for a former politician. So possible exile? Certainly.

However, the reason Caesar's enemies would drag him through the courts was far deeper than just him doing whatever he wanted to - Cato Minor, of course, used the war in Gaul as an excuse, but that was far from the only reason. Caesar was just too powerful at that point - he had a heavy arm of support, and, most notably, like literally every other Roman politician of the time, his campaign was incredibly corrupt. His war in Gaul, while treading the borders of legality (He WAS the proconsul of Gaul, and the Gauls WERE rebelling. Or asking for his help. Or being Gauls, and Gauls HAD razed Rome 340 years prior. Totally good reasons!), wasn't as significant as that corruption. The Romans nominally despised corrupt elections, despite the fact that it was impossible to be elected in the late Republic without bribing a huge number of people.

Here's essentially the timeline of dominoes:

• Cato throws a tantrum over Caesar's war and demands that he be handed over to the Germans for justice. He's essentially laughed off.
• The Triumvirate is renewed - Pompey, Crassus, and Caesar hold essentially complete authourity over Rome
• Double death of Julia and Crassus. The former undermines the relationship between Caesar and Pompey, the latter shakes the Republic itself. As incredibly wealthy as Crassus was, a truly absurd number of people were his clients.
• Caesar wished to become consul again in 49 BCE, 10 years after his first consulship, as was traditional. However, his actions during that first consulship were still considered to be illegal (especially his corruption on going into it), and he'd only dodged the courts by being in office throughout the intervening ten years. With that power, he would be able to fix all of the political snags in Rome, navigating past his enemies and making himself so strong that they couldn't risk attacking him in courts.
• Above all, Caesar was striving to protect his dignitas - a HUGE concept to the Romans. If you'd like an explanation of that, I'll be happy to provide it - because, in the end, Caesar's Civil War all boiled down to one man trying to protect his dignitas.
• The Romans were HUGELY averse to any one man having too much power. Pompey had legions right outside Rome and was seen as too powerful to move against - plus, he was more amenable to the Senate. Caesar, on the other hand...
• Caesar was also accused of keeping his troops past their discharge dates (he had).
• Domitius Ahenobarbus wanted to prosecute Caesar for his consulship.
• Cato took an oath to prosecute Caesar for the same thing (They also considered the laws he had passed to be illegal)

TL;DR - It's way more complex than that, but the trial would mostly have dealt with the events of 59 BCE - Caesar's consulship.

One final note - your final sentence is a bit choppy. Try something more like "He returned to Italia with one legion under arms, sparking his civil war against Pompey."

Hope that helps!