To add to the existing answers:

Why kill a prominent member of the community? That would spark investigation into the murder. A risk which Voldemort was probably not willing to make.

Voldemort did kill, and almost spontaneously sometimes. But consider this statement from HP7 (Bathilda's Secret) when Voldemort is approaching the Potters':

“Nice costume, mister!”

He saw the small boy’s smile falter as he ran near enough to see beneath the hood of the cloak, saw the fear cloud his painted face: Then the child turned and ran away. . . . Beneath the robe he fingered the handle of his wand. . . . One simple movement and the child would never reach his mother . . . but unnecessary, quite unnecessary. . . .

-Harry Potter and The Deathly Hallows, Chapter Seventeen (Bathilda's Secret).

He did not kill if it was unnecessary, which killing Slughorn would have been [I believe].

To add to the existing answers:

Why kill a prominent member of the community? That would spark investigation into the murder. A risk which Voldemort was probably not willing to make, since he was still in hiding at this point; and was still able to use the element of surprise and drive his machinations from afar, without drawing too much attention to himself.

Voldemort did kill, and almost spontaneously sometimes. But consider this statement from HP7 (Bathilda's Secret) when Voldemort is approaching the Potters':

“Nice costume, mister!”

He saw the small boy’s smile falter as he ran near enough to see beneath the hood of the cloak, saw the fear cloud his painted face: Then the child turned and ran away. . . . Beneath the robe he fingered the handle of his wand. . . . One simple movement and the child would never reach his mother . . . but unnecessary, quite unnecessary. . . .

-Harry Potter and The Deathly Hallows, Chapter Seventeen (Bathilda's Secret).

He did not kill if it was unnecessary, which killing Slughorn would have been [I believe].

To add to the existing answers:

Why kill a prominent member of the community? That would spark investigation into the murder. A risk which Voldemort was probably not willing to make.

Voldemort did kill, almost spontaneously sometimes. But consider this statement from HP7 (Bathilda's Secret) when Voldemort is approaching the Potters':

“Nice costume, mister!”

He saw the small boy’s smile falter as he ran near enough to see beneath the hood of the cloak, saw the fear cloud his painted face: Then the child turned and ran away. . . . Beneath the robe he fingered the handle of his wand. . . . One simple movement and the child would never reach his mother . . . but unnecessary, quite unnecessary. . . .

He did not kill if it was unnecessary, which killing Slughorn would have been [I believe].

To add to the existing answers:

Why kill a prominent member of the community? That would spark investigation into the murder. A risk which Voldemort was probably not willing to make.

Voldemort did kill, and almost spontaneously sometimes. But consider this statement from HP7 (Bathilda's Secret) when Voldemort is approaching the Potters':

“Nice costume, mister!”

He saw the small boy’s smile falter as he ran near enough to see beneath the hood of the cloak, saw the fear cloud his painted face: Then the child turned and ran away. . . . Beneath the robe he fingered the handle of his wand. . . . One simple movement and the child would never reach his mother . . . but unnecessary, quite unnecessary. . . .

-Harry Potter and The Deathly Hallows, Chapter Seventeen (Bathilda's Secret).

He did not kill if it was unnecessary, which killing Slughorn would have been [I believe].