It's more about inconstancy. And she miscalculated. She expected Aarya to take her union with the homunculus and her daughter so badly, considering she wasn't some stranger, while Aarya had absolutely no problem with humans when they were alone. Narrative logic died somewhere here.
Everything looks as if Nidhogg died by accident and Harald simply decided not to let the good things go to waste. And when his action was revealed, he was like “oops, sorry, but I really needed that.” In this case, I would probably really just shrugged, but we are dealing with premeditated murder.