Orgetorix was an helvetian aristocrat that attempted to migrate his people from Helvetian territory to south-western Gaul, right before Jules Caesar gaul campaings.
He was also party to a clandestine arrangement with Dumnorix of the Aedui and Casticus of the Sequani to seize control of their respective tribes by arms and between them rule most of Gaul. The conspiracy was denounced, Orgetorix was called to a hearing in chains before the government of the Helvetii, he arrived with a small army, and was released, but died mysteriously, a rumored suicide.
What really happened? And what are the historical sources confirming his death by suicide?
People searching to overthrow constitued institutions in ancient Gaul at their benefit, or even only suspected attempting so, were risking capital punishment.
The problem wasn't about legitimacy as it's likely that Orgetorix was of royal lineage, something that certainly helped them raise an important clientele if it was the case. The problem is that he attempted a coup : worse, he attempted to do so with foreign support; and not just as a leader of a faction but by taking a legal office and using it for his own ambition.
Gauls, as the other ancient peoples with sophisticated political institutions, didn't look kindly on this : Celtillos (proably descendent of Areuernoi royal line) was executed being accused doing so, Tasgetios king of Carnutes more or less put by Caesar as a client was murdered (while Carnutes had maintained a royal authority, Gaulish kingship wasn't sacralized, and rather magistral), etc.
Giving that the penalty that Orgetorix was to be condamned to among Eluetioi was being burned at the stake, he attempted first to pressure the assembly by gathering his armed supporters, which allowed him to avoid the trial by force (and not released, which would imply the trial would have been dismissed or achieved) but Eluetioi citizens armed and hunted him until his death.
While an execution or a murder is possible, there's no real evidence that the rumored suicide didn't happen : Orgetorix simply went too far, was certain to be cruelly executed, and suicide might have been an honorable and less painful way out.
There's enough exemples among Greeks and Romans politicians and chiefs, and of suicide of defeated Gauls that there's no particular reason not to think Orgetorix couldn't have done so.