This is literally the sweetest thing omg
Honestly at first I struggled a lot with both Scara and Childe. They're very complex characters in that they're made up of these seemingly completely opposite/incompatible elements - being this sweet, loving guy but also a bloodthirsty weapon; or in Scara's case, someone who (in his opinion) is above every mortal being and would do just about anything to anyone to get what he wants, but somehow also (sometimes) is so deeply kind.
If we're being super honest, I identify way more with Scaramouche, so it's possible that's why his voice comes naturally to me haha

But on a more technical level, I think it helps to keep in mind that (oddly) the core of him is just...yearning. Not in a "simp" way, but in the sense that his whole identity is crafted around this feeling of emptiness and feeling incomplete. /Everything/ has to come from there. Is he being an asshole? Well in his mind, he's missing the part of him that wants to be nice (and the part that deserves to be liked/loved - so doesn't it make sense to hurt people and push them away?).

Similarly, when he does reach the point of actually wanting to connect with someone like he does with Childe, it's overwhelming because there's this realizing that that connection will fill in the void he's been trying to fill on his own. And that's not necessarily a good thing - because if your whole identity is based on being empty and superior and unlovable, what happens when you fill that space with love from another fragile, fallible person?

...I'm not sure how much any of that made sense, but it's what's in my head when I'm writing! I go back and adjust his internal monologue quite a bit to make sure it's balanced once I've got the shape of a scene down, as well. It's really mostly instinct at some point.