House Elf Orders
I have a couple quick questions about house elves.
If house elves receive conflicting orders from two masters, whose do they follow (e.g. one says clean up the dishes and another says leave the dishes on the table)?
Also, can a house elf kill one of its masters and not hurt itself (e.g. if one master orders a house elf to kill another master, does the house elf then have to punish itself after commiting the deed)?
I think that the most recent order would probably clear out the first, if the commands came from people with equal power. For instance, if Mrs. Black said "clear the table, Kreacher," and Sirius conteracted that with "no, leave it there," Kreacher would probably continue following Mrs. Black's commands. But if Mr. Black told him to leave them there, he would probably go with the most recent command.
Another thing to consider would be if the house elf has a favorite. For instance, Mrs. Black had Kreacher's absolute loyalty, even in death, so he's more likely to follow her commands.
As for the killing thing, unless the master he killed gave him the clear command of "do not kill/injure me," then the house elf wouldn't have to punish himself.
Hope I helped!
Like Kate said, it probably has something to do with the house elf's favorite when they choose which command to follow. It could also have to do with which member of the family with the most power. Dobby would probably have listened to Lucius' commands if they conflicted with Narcissa's or Draco's, and even though Kreacher's favorite was Mrs. Black, he might have had to listen to Mr. Black's orders if they conflicted with hers.
House elves only have to punish themselves when they disobey their masters. For instance, Dobby had obviously been ordered not to leave the house when he warned Harry not to go to Hogwarts in the second book. If Lucius had never told him to stay put, Dobby might have known that his master wouldn't like what he was doing, but he wouldn't have to punish himself because there was no direct order.
So basically if there was no direct order not to kill the master, the house elf wouldn't have to punish themselves.
Hope this makes sense/helps!