Human "alien hunter" meets a real alien

Question

This is a short story (only a few pages as I recall) probably from Asimovs in the 1980s. The main character is a man who looks for aliens in human disguise and kills them, secretly. The story opens with him at a bank (I think) noticing a woman he suspects is an alien nearby. He suspects her among other things because her line is moving faster than his. He follows her out of the bank and corners her - and she mentally reveals that she is an alien. She taunts him with her ease in fending him off, and he realizes that the others he killed must have been humans (since he was able to kill them) before she kills him.

Answer 1

This is "The Protector" (1982) by Art Vesity, published (so far) only in Isaac Asimov's Science Fiction Magazine, Mid-December 1982.

The story opens with the protagonist, Tanner, in a bank surveilling a couple:

He stands at the high marble-top desk in the First National Bank, pretending to fill out a deposit slip. Out of the corner of his eye, he watches the two people conferring—no, conspiring—near the elevator. The woman is petite and has tangled blonde hair of short length. She wears tight, faded jeans, and next to her is a stroller containing one sleeping baby. The man is of medium height and build and wears a gray pin-stripe suit that fits perfectly.

He actually expects them to be able to make their line move faster:

Tanner waits for the odd couple to pick a line, then joins the other. You’re lucky, he thinks to the people in the odd couple’s line. You’ll be out of here in no time.

As Tanner expected, their line begins to shrink. For no apparent reason, the teller, a pretty young woman with soft blue eyes, begins to work faster. First, the young man with whom she had been flirting is dispatched. Then, a broad-shouldered black man in a bus driver’s uniform. And next will be the middle-aged woman in white. Hairdresser? Nurse? A spy in league with the odd couple? Tanner cannot see her eyes, or the eyes of the others, but he doubts they are spies—there are rarely more than two in any one vicinity. But the line is moving faster than his now, and the people ahead of Tanner grow even edgier and eye the other line with envy.

He doesn't immediately follow them, he finds her deposit slip which has her address:

Tanner grins. She is no spy. And like the others he has questioned today, at the supermarket, the drugstore, and finally here, the odd couple’s most recent stop, she had not noticed that this time her line moved faster than the other. Something, someone has affected their minds. The odd couple, no doubt. The man and the woman.

The alien spies.

Suddenly they emerge from the bank, not noticing Tanner, and go in separate directions without saying goodbye. Unknown to the woman, a slip of paper falls from its precarious perch at the mouth of her jacket pocket. Then, to further confirm that the fates are with him (and thus, with Earth), the wind picks up the paper and blows it to within inches of Tanner’s feet. He picks it up with a steady hand and reads it. A deposit slip, with account number, name, and address.

Tanner smiles. This time, it will be easy.

But of course it's not; she's prepared for him and far too powerful:

"It would seem that we two have been touched by the long arm of pure, unaltered chance, Arlen. Very well, then. I suppose there’s no talking you out of it... Go ahead and shoot."

Do it, says the Voice. Don't let her calmness fool you. Complete the mission, immediately.

Thus reassured, Tanner aims at her heart, pulls the trigger... and misses. Not entirely—as the dull "pop" is heard from the silencer, the alien grabs her left shoulder and winces in pain—but he has missed her heart. A second shot will be necessary, for the first time in his career as Protector. He fires again, but this time the bullet misses her entirely and strikes the edge of the bedroom door-frame, splintering it loudly. This isn't possible, Tanner thinks. It's nearly point-blank range!

"No more bullets will strike me, Arlen," says the alien. "I can direct them away with little effort. I allowed one to strike me in an easily repaired area, because there is something I wish you to see."

Tanner feels himself shaking uncontrollably. He leaps toward her, presses the gun into her stomach, and tugs at the trigger repeatedly... but the gun is jammed.

"The gun will not work, Arlen. Now, look."

She reveals that he has been killing real people:

As she takes her hand from the wound, Tanner steps back, aghast. A thick, greenish-yellow liquid is oozing slowly from the hole in her dress. "Behold my blood, Arlen," she says softly. "And now tell me—what was the color of the others' blood? What was the color, Arlen?"

He feels his face freeze into a fools mask, mouth and eyes wide open, head shaking from side to side in childish denial. Red, says the Voice, but it sounds different now, more like his own, more like the old voice that used to haunt and mock him before he became a Protector. Red was the color, red red no yellow but no blood is yellow. Aliens lie deceive the Voice was wrong and oh God the motherless children...