It is vital of you to understand that Illya is not a child. She roleplays as one when it’s convenient to her (namely to either fuck with Shirou and/or pretend she is in a normal family for a bit) but she’s not actually a kid. She’s an extremely maladjusted adult who was raised in debilitating social isolation for one specific purpose, which means she got the combo of “no actual moral compass, just a duty to fulfill” + “no fucking baseline for what normal human interactions are like” + “is going to die soon and knows it so who cares about consequences” + “has been constantly denied basic agency her whole life and even now that she’s out of the Einsbern mansion people keep treating her like a kid.”