Assassin (in Short Science Fiction Collection 061 )

The aliens wooed Earth with gifts, love, patience and
peace.
Who could resist them? After all, no one shoots Santa
Claus!

The rifle lay comfortably in his hands, a gleaming precision
instrument that exuded a faint odor of gun oil and powder
solvent. It was a perfect specimen of the gunsmith's art, a
semi-automatic rifle with a telescopic sight--a precisely
engineered tool that could hurl death with pinpoint accuracy
for better than half a mile.

Daniel Matson eyed the weapon with bleak gray eyes, the eyes of a
hunter framed in the passionless face of an executioner. His blunt
hands were steady as they lifted the gun and tried a dry shot at an
imaginary target. He nodded to himself. He was ready. Carefully he
laid the rifle down on the mattress which covered the floor of his
firing point, and looked out through the hole in the brickwork to the
narrow canyon of the street below.

The crowd had thickened. It had been gathering since early morning,
and the growing press of spectators had now become solid walls of
people lining the street, packed tightly together on the sidewalks.
Yet despite the fact that there were virtually no police, the crowd
did not overflow into the streets, nor was there any of the pushing
crowding impatience that once attended an assemblage of this sort.
Instead there was a placid tolerance, a spirit of friendly good will,
an ingenuous complaisance that grated on Matson's nerves like the
screeching rasp of a file drawn across the edge of thin metal. He
shivered uncontrollably. It was hard to be a free man in a world of
slaves.