My heart beat high now at the chance of action,
and drawing the sharp sword from my hip i went
along his flank to stab him where the midriff
holds the liver. i had touched the spot
when sudden fear stayed me: if i killed him
we perished there as well, for we could never
move his ponderous doorway slab aside.
so we were left to groan and wait for morning.

what prevents odysseus from killing the sleeping cyclops?

he thinks he can reason with the cyclops in the morning.
he wants to make the cyclops his ally and friend.
he knows that they cannot move the boulder blocking the doorway.
he feels sorry for the cyclops who lives all by himself.