Just an update, my ticket has now been escalated; I'll reply within this post what the GM response is. Here's what I'd written:
Horde shaman ### killed rare spawn Karkin while myself and three hunters were attempting to tame. Has apparently done this before, and spoke to us very rudely after killing it. Other hunters very upset; by this point I've come to expect this of people, but the other hunters were very distraught. Please investigate this player and inform them that they are disrupting game play of multiple people. Thank you for your help.As icing on the cake, as soon as I logged in to Molten Front, I read this in General:

It's odd how much class hate is going on this patch. I agree with AdamSavage that we just need to keep reporting, because the amount of griefing and bullying going on is, in my experience, unprecedented. It especially irks me when I know the fellow rare-campers are younger kids and take this to heart, or are recuperating from necessary medical procedures. I acknowledge we do get a rich cross-section of players in this game, and will always have trolls and what have you. Some people just missed out on the whole respect thing in life, and need to be slapped on the wrist every now and then. As Worba said above, report them and move on.

EDIT UPDATE
GM response to my ticket below:

Very well handled and an extremely fast response time!

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