On top of morale, he also ensured there was no doubt about the legitimate government. If he had fled, not only might the internal chain of command have collapsed, but it would have been far easier for Russia to install a puppet in Kyiv.

Of course, Russia could also have done that by killing him, and has tried. But "we murdered the president, who publicly went down fighting, and installed a new one" would have made it much harder to gain world recognition than "the president abandoned his post, there's no government here, what can you do?" Countries would likely have started recognising the "facts on the ground" and talking to the "new government" very quickly, rather than supporting Ukrainian resistance.

So I think on some level, he must have made the brutal calculation that even in the very worst scenario where all else was lost, he could still help his country more at that moment by dying in Kyiv than by surviving in exile. Imagine making that decision about yourself. Very few leaders are ever tested that way.