In addition to all of that,
Aster viewed Aria as a threat. Aria wouldn't have agreed with the whole murder aspect of his ideology, AND she was set to become queen of the kingdom (while also being the daughter of a major noble house). She could have easily opposed him. By framing + enslaving her, either she gets convicted and is removed as a problem, or gets off and is still under his control.
And it would've worked with Celeste's true parentage giving Lucio an out.