He wasn't sure to die. There was a force of somewhere between 6000 and 11,200 soldiers stationed there (of which 300 happened to be Spartiates). Nearby at Artemisium, there was a fleet of 280 ships and 50,000 men, designed to block and (hopefully) destroy all sea support for the land army. And the invading land army sure as hell needed a lot of support, because it was huge and there was no way it could sustain itself off the land.

For comparison, the oriignal plan had been to place the defensive line at the valley of Tempe, further north, manned by 10,000 men. The defensive force at Thermopylae was of a comparable size.

And, without the benefit of hindsight, there are arguments in favour of Thermopylae being potentially winnable. We don't know how large the invasion force was -- the figures given in Herodotus are ludicrous: 1,700,000 Persians and 300,000 Greeks -- but even if that's exaggerated by a factor of 20, that's far too big an army to live off the land. Eight years after the battle, the playwright Aeschylus wrote

The land itself is an ally to [the Greeks] ...
killing with hunger an army that is far too numerous.
(Aeschylus, Persians 792-794)

Chris Matthew (Matthew and Trundle, eds. [2013], Beyond the gates of fire, pp. 60-99) suggests that the idea of the defence of Thermopylae wasn't just to block the pass, but to trap the northern army in a position where it had no access to food or fresh water. If the southerners could block or destroy the invasion force's naval support, and delay the land army for even a week, and reinforcements came via another pass and appeared behind the northern army, then Xerxes’ forces would be trapped at the very moment that they were weakening from hunger, thirst, and disease. They would have been annihilated.

The problem is simply that they couldn’t hold the northerners that long, as it turned out. Leonidas only managed three days. And that wasn’t enough.

When Leonidas ordered a partial withdrawal, it's possible that at that point he was expecting to die holding the line. But given that the remaining force was trapped because the Persians scaled the hills and trapped the southern force, it seems much more likely to me that they were intending to withdraw too. The just weren't quick enough. And so the remaining force -- 700 Thespiaeans, 400 Thebans, several hundred Laconian perioikoi and helots, and 300 Spartiates -- were caught with no way out, and were cut down.

By the way, there's no reason to imagine that the 300 Spartiates were his 'best men'. They weren't his personal retinue or anything like that. 300 was a typical number for a Spartan force on a specific assignment.

There is a very informative AH podcast about this: https://open.spotify.com/episode/73yXmggd14eMHWY9d7pXBi?si=v0C0c70yRqegiYuptlrq8A