The poem is two short sentences long, and the second one changes everything. The first sounds tender: a goodbye kiss, and a very sweet one. Then the speaker compares it to antifreeze, which really does taste sweet but is deadly to swallow. That single comparison turns a romantic memory into a warning.
The speaker seems to be looking back on a relationship that ended badly, or on a parting that only felt loving at the time. The kiss may have been a performance, a way of softening a cruel exit, or a gesture that now looks false. The repeated word "sweet" makes the bitterness sharper, because the speaker is almost mimicking the charm they once believed in.
The mood is cold, wry, and a little stunned. There is no shouting here, just a calm, clipped tone that suggests the speaker has worked out what that sweetness cost them. It's a small poem with a sting in its tail.