Just a small tad of an addition here . . . There may have even been a plan for Snape to take the time to pretend to "gloat" over the chance to kill DD. But I think circumstances were a little more dire at the key moment than they had anticipated. Dumbledore was near death as it was from the potion. I think the pleading may have been a cue to Snape (possibly non verbaly as well) that he better get it done now or he was going to die anyway, and they would have lost any "show" for the surrounding DE's to strengthen Snapes position with Voldemort. I think that someone else in the order may have been informed of the arrangement between DD and Snape, and that will likely come out at some point early in book 7 to put Harry right . . . but . . . he may likely still reject it. Harry will likely find out when perhaps Moody (suspiciously absent in HBP) gives Harry some info on Voldemort's plans and must explain where the info is coming from . . . OK straying off topic here LOL.