Dumbledore didn't plead for mercy

I actually did intrepret it as a plea for mercy -- because giving Albus a swift and painless end was mercy, compared to the effect of the poison he drank.

And I find it actually supports Snarry, in a way -- Severus and Harry were both agents in Albus' death, Harry with a goblet and Severus with his wand, both obeying Albus' own orders. The connection between them, if Harry ever is able to learn/accept the truth, is stronger than ever.

:;tries to shush the plot bunnies in her brain::