Lyric discussion by Ckp2ator 

Cover art for Alison lyrics by Elvis Costello

I thought of it as a guy runs into a past GF. By the line she "was not impressed" by the look she gave him, I interpreted it as she had married up, maybe she's now a wealthy suburban matron. She's changed and he's sick of hearing all the "silly things" that she has to say. Such that he wants to turn off the light, i.e. end the conversation. He can't stand her new fake self, and her new life is killing her true self. He concludes with "my aim is true", that is, his assessment is right.

[Edit: clarity]

Negative
Subjective
Disgust
Past Relationship
Resentment
Change
Disillusionment
Truth