A pair of alien races have decided to settle rival claims on the Earth by agreeing to a competition between their best agents. The competition is a deadly one, with the winner of the competition being the one who kills off the other side's agents first.
Each side is allowed to send (IIRC) two agents, disguised as human beings. The only way to see the respective aliens as they actually appear is to view them through a special set of glasses (this isn't They Live by John Carpenter, btw - much earlier film).
I cannot recall the name of the two alien races, but I do remember that one was a benign society, while the other was treacherous and militaristic. The idea is that Earth will have to submit to one of the two at the end of the contest, but one alien overlord would clearly be preferable to the other.
The preferred weapon of both sides is some sort of ray gun/death ray pistol. When an agent has been slain, some sort of device is placed on their body and activated. This vaporizes the body completely.
The protagonist of the film is a male agent from the "good" alien side. He's killed one of the enemy already, and is tracking the remaining baddie. The enemy aliens have eliminated his compatriot, so it's now him or the bad guy.
Somewhere early in the film, the protagonist meets up with an Earth woman. I do not recall the circumstances of the meeting, but they become friends and perhaps more. At one point, the protagonist explains the difference between the two alien races, and also allows the woman to see him through the special glasses. He insists she take a sedative first, as his true form might be too much to take. She faints after seeing him (the viewer is never shown this form).
The film seems to end with the protagonist triumphant, with the elimination of the last enemy agent. Then we are treated to the twist. The Earth woman is actually a member of the enemy aliens. Her side cheated and sent an extra agent.
She shoots the protagonist and he begins to die. Fond of the protagonist, she is remorseful that he is going to die, but tells him "we had to do it, don't you see?" or something like that. He dies and she disintegrates him with the device.
The final scene is a zoom in on the special glasses and we finally see what one of the aliens looks like. I seem to recall it being an animated, glowing figure, though I'm not certain of that.
I saw this film in either 1979 or 1980. It was in color, and it may have been a made-for-tv movie, though I don't think it was. I would guess the film was made between the mid-60s and late-70s.