[Zigûr]

It's become my impression that Gil-galad and Elendil severely wounded Sauron, perhaps fatally, Sauron burned Gil-galad to death with his hand and struck Elendil down, Elendil fell and Narsil shattered beneath him, and Isildur took the hilt and dealt the final blow to the crippled, unconscious or otherwise incapacitated, possibly dying, Sauron by hewing off his finger.

This is to say, I feel that Isildur didn't wield the broken hilt of his father's sword in some daring act of martial prowess but rather as the best instrument he had to hand for the blunt act of amputating his enemy's digit.

"Was it not I that dealt the Enemy his death-blow?" Unless Isildur is lying, I feel like this explanation is at least one which reconciles the three ideas we hear in The Lord of the Rings and The Silmarillion that:

a) Elendil and Gil-galad killed Sauron
b) Isildur "dealt the Enemy his death-blow"
c) Isildur used the hilt to cut the Ring from Sauron's finger

b) and c) might be separate incidents, however: maybe he stabbed Sauron with the hilt first or something to that effect and then severed his finger. I've found this bit confusing for years.