Clara started out trying to avoid her "villainess" route like your typical reincarnated person, however after living in the world, she came to actually fall in love with her fiancé. This leads her to do things that her game counter part would do, like sending assassins, directing a hoard of demons towards the capital, etc. She even muses at one point that the game version of herself that she knows may not have been a different person at all, and may have just been her to begin with.
A bit of a preface to this part, but Carla is my favorite character, so I will gush a bit about her. Carla as stated before is a daughter of a well known family that produced strong magicians for generations. However Carla herself was born mostly normal, as in average abilities. Her father then experimented on her which gave her the ability to use all attributes but since having more magical attributes cuts down on your life span, anyone can guess what happened. After she "broke" like Elena she was to be cast out as a failure, however she is still basically a monster and kills 2 of her old brothers and threatens her father who decides to bide his time and use her for his advantage. Carla has since lived her life with one goal in mind: Destroy her family and kingdom in the most horrific way possible so that not only will they no longer exist in the future, but stain any honor, pride and legacy they had. That is until she met our MC, Aria. Now the narration for her is closer to a love struck girl. In the game that Clara knows, Carla was obsessed with the prince, in a "I want to break that pureness" type way. This Carla loves Aria, she loves her stoic loner aura, she loves her unrelenting drive for power to fight her fate, and above all, she loves Aria bathed in blood. All of her previous motivations just become pale in comparison to her obsession with Aria, all of it becomes things that she wants to set up for their inevitable fight. The stage she envisioned she would stand with Aria is that of the kingdom on fire, rivers of blood flowing, the broken mangled bodies of the kingdom's leaders (her father, the king, the prince, etc) before them as their audience. And finally, Aria's dagger buried in Carla's chest, as Carla's blood covers Aria and Aria's blood covers her. Carla will kill Aria, and Aria will kill Carla, that is the future she will strive for no matter what she will has to pay for. Whenever Carla and Aria meet throughout the story, it is nearly always with Aria asking "Is it time?" to which Carla will reply "The stage isn't ready yet.", they then trade info and possibly act together for a common goal. To put into context how the scenes normally play out, we'd have Carla gently approach Aria and place her hand on her face, before using her nail to make a cut on Aria's face so she'd draw blood, and Aria would place her hand on Carla's neck, ready to crush it at a moment's notice. All this time Carla stares longingly into Aria's eyes imagining their "ending" together. To simply put Carla's mindset can best be summed up as "ARIA ARIA ARIA ARIA ARIA ARIA ARIA ARIA more power ARIA ARIA ARIA ARIA ARIA ARIA".