Why didn't he flee? This was a long time coming
If he decided to stay for ideological reasons, they would not have been there.
My guess is that they might have misinterpreted the US's demands as starting positions while the US considered them to be final. Who would expect a country that can produce ballistic missiles to willingly give it up? It was a non-starter from the beginning.
For what it's worth, I agree with you about Sinwar dying while fighting.
Tell that to the soldiers in the famously almost universally ineffective militaries of almost all the countries in the Arab world.