Maxperson
Morkus from Orkus
I think the idiot deserved to die. But the person asking for mercy shouldn't have had to roll anything. At worst the result of that roll should have decided the fate of the rogue.
Beyond that the details are a little light. From what I can tell it's not that "the party" attacked or antagonized the dragon, it was one individual who apparently had a death wish.
Guilt by association. If it hadn't been a drow, I wouldn't have quite as strong of an opinion about what the dragon did. If a player chooses to play a race with the evil reputation of the drow, and the party lets that drow in, it will have a negative effect on things. Good people don't associate with evil drow. Evil ones do. Once the drow acted in the manner it did, confirming through his actions that he is evil, the dragon was free to act in the manner it did. Perception is greater than reality and the perception of the dragon about the evil drow taints the rest of the party and its perception of them.
Last, but not least, people play for a lot of different reasons. In this scenario, if I were DM the dragon would have ignored the plea and killed the rogue. After that? He'd let the party decide what they were going to do now that the nuisance was taken care of.
For me, if the rogue hadn't been a drow I would have done that.
Agreed/On the other hand if the group had fun, it's the right DM for that group.