In general, the idea of a parlay where both parties can feel safe and negotiate, even if they are ultimately likely to fight, is a very pervasive one. There certainly are cases where people were killed during negotiations and meetings like this, but in a lot of cultures this is regarded as a huge taboo, for the simple reason that a breakdown in the trust around negotiations would make ever negotiating any sort of peace settlement impossible, and would make future wars much more difficult.

If Henry had killed the Dauphin at a negotiation (as an example, I have no idea if they ever actually met like that, or if that is dramatic license on the part of Netflix), he might have succeed in ridding himself of one enemy, but would have likely made many more, and those enemies would have nothing to lose by fighting him to the bitter end. Its the same idea behind the whole 'Red Wedding' thing in Game of Thrones, if lords couldn't feel safe meeting with each other, this was seen as one of the worst taboos imaginable, because it undermined the whole basis of how diplomacy and politics was supposed to work.

A famous slightly grey area example I can think of is Wat Tyler, leader of the peasant revolt against Richard II. He was killed during negotiations with representatives of the King after a fight broke out, though by some accounts Tyler drew his weapon first. In this case, it did succeed in ending the revolt and given that Tyler was a rebel and it could be argued he started the fight, this didn't really shame Richard in the way one might imagine, as far as I know.

When the victims of such dishonourable acts were not viewed as rebels, however, the consequences could be dire. A famous example would be the killing of a Mongol envoy by the Khwarezmian empire, which resulted in a particularly brutal campaign of reprisal by Ghengis Khan.

Or the Glencoe Massacre in Scotland, while not exactly violating a parlay, is a famous example of a similar violation of the norms of safe passage. Following the ascension of William III in the Glorious Revolution, troops from the Campbell clan loyal to William were billeted with the clans MacDonald and Henderson, who refused to swear an oath of loyalty to William. This caused tensions with the Campbell troops which erupted into a massacre where they killed dozens of their hosts. While similar episodes of clan violence, and violence linked to loyalty to the Stuarts v William and subsequently the Hanoverians, were fairly common, Glencoe has been remembered in a way the rest has not, because it involved betrayal of one's hosts, which was seen as an absolutely horrendous crime. The association of the Campbells with this betrayal persisted for hundreds of years afterwards, and is genuinely still sometimes invoked today, though mostly now as a joke.

Short answer: He could have killed the Dauphin, but it likely would have been far, far more trouble that it would have been worth.