Sorry that was perhaps a bit short and vague.I also think the word pet doesn’t work well in this context, in the death scene of a tragic character and coming from the mouth of a balrog.
What I mean is, calling Edhellos pet does work by degrading Edhellos and expose her as someone who has had submitted to Sauron, but 1) in this scene, in which Edhellos will die, this smearing risks tainting her end and setting the tone of her death, and working against her redemption, and the feeling her husband, and others, will have of her after her death. To me, calling her 'pet' implies a more willing deference, someone who is tamed, while 'slave' implies that a person has been chained or broken but needs to be reminded of it from time to time, and most importantly, someone who performs certain tasks - in this case, betrays her people. Calling her 'slave' works better imo since it implies a betrayal while simultaneously allowing for redemption. 2) The Word 'pet' is full of sarcasm, which is a weapon fitting for some of Morgoth's people, but perhaps more something Sauron
would say (or later, The Mouth of Sauron). I Believe Gothmog is cruder.