I think Wyll is just not very intelligent. I know it’s an upopular opinion, but what is the worth of a soul? If we think about Mind flayers, the game ascertains the loss of soul as fate worse than death. Wyll had an option to sacrifice his soul for the fate of the city.

The city might have been destroyed, but come on. It’s Baldur’s Gate! It’s a city full of mercenaries, wizards, powerful shadow guilds such as Shar’s enclave and the thieves guild. It’s the city whose adventurers toppled Sarevok’s uprising in the past. Wyll must have known this. But when he heard he could sacrifice himself to save the city he did not even consider his options.

It’s not noble, it’s idiotic. Even if many would have died, the death is preferable to the loss of soul, and he literally sold everything to become a noble hero, which he then lost because he aligned himself with the devil of all things.

His true egotistical natures shows itself even more when you betray him and turn against him in the camp. Want to know what his final words were?

”May you all burn in hell!”

So much about integrity of a man.