Julius Caesar's reputation for being magnanimous is somewhat overblown.

He could certainly be magnanimous when the enemies were fellow Romans, and there was a political benefit to be mined from it, but if the enemies were foreign and there was no political benefit from showing mercy, or he instead benefitted from being ruthless...he was ruthless.

There are plenty of examples of Caesar being brutal with his Gallic enemies. Avaricum for example, where a city of some 40,000 was put entirely to the sword, or the aftermath of Uxellodunum where Caesar ordered the hands of all the Gallic prisoners lopped off, before scattering those prisoners throughout Gaul, so they would be a demonstration of the price of raising one's sword against Rome. Going father back in his career, he also was quite ruthless in his retaliation against the pirates who had held him for ransom. Execution for pirates was not necessarily a given, as Pompey for example famously spared many of the Cilician pirates following his suppression of them, pardonining those who had turned to piracy out of desperation due to poverty, and resettled them in cities. In short the execution of Vercingetorix was not really out of character for Caesar.

It's impossible to say of course what motivated the decision not to show clemency, as his thoughts on the matter were never recorded. I do recall watching a BBC documentary about Alesia ages ago where one of the historians on the programme speculated that Caesar probably knew Vercingetorix personally prior to the rebellion, as Caesar frequently met with tribal leaders during his campaigns & Vercingetorix was an important figure among the Arverni, who were also one of Gaul's most powerful tribes. He posited that the reason for Vercingetorix's execution my have been personal rather than political - that Caesar was angered that Vercingetorix had pulled the wool over his eyes. Again though, that was just speculation.