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].