I feel like it’s obvious that Gia was 100% revenge. Only difference is that, like with his punishment for Hayley and all his wrongdoings towards everyone, he can justify it by pretending that he had to do it. Now the Marcel’s minions that walked in were collateral damage. But Klaus purposely singled her out to kill her in front of Elijah’s face. At least everyone else’s punishment was temp. Hayley’s curse can be broken, and Bekah still got to have her bodies and didn’t actually die,
but Elijah’s was permanent. He can’t get Gia back, not to mention him have the image of Gia burning before his eyes. If it had been just for the plan, there were plenty of other ways to achieve that goal. So he didn’t kill Gia because he had to, he did it because he wanted to. & that’s why I found Cami’s (not total) lack of empathy a bit disturbing. & she saw how angry Elijah was, she saw Gia’s body. & yet she still defended Klaus. I wonder if she would’ve done the same had he killed Aiden.
It’s impossible to know how much she knew either of them because I don’t think she had any scenes with Gia, and only a handful with Aiden, but considering she’s a bartender and Gia was Marcel’s friend, she probably met her at some point. But Aiden, Josh and Davina were together a lot, and she’s close to Josh and Davina, so I think it’s fair to assume she hang out with Aiden from time to time.
Gia walked in and he would have to kill her anyway because it was part of his plan, it wasn’t right, but he was killing all of Marcel’s minions. But the part where he saved her to kill in front of Elijah, that was revenge, I would say it was part of the plan if it was Aunt Dahlia’s suggestion, but it was his choice, she would have been fine if he had killed Gia in Marcel’s loft, he took her to Elijah because he’s a douchebag.
I wish Cami had more empathy towards Gia, but I can also understand her reasoning, in her mind, it was all part of the plan, it could have been handled better, but it was a terrible path in hopes to get what you want in the end. We saw her do terrible things to try to save Kieran, including turning him into a vampire. I think that she can relate to doing awful things to reach your goal.
But she couldn’t make sense of why he killed Aiden because it would have been 100% out of pettiness, because Aiden dared to defy his order, it didn’t achieve anything, it wasn’t part of a bigger plan.