when she was being such a bitch about not even being able to wait a single day. It's obvious that she cares less about him since she didn't even remember it was their six month anniversary. Her actions and lack of trust do not bode well for a long term relationship.
Did you even read the newest chapters? He was being suspicious seeing your man outright lie to you will make you anxious! Of course she was scared and wanted to know the truth and not all does the half year anniversary thing Masugu wasn't planning to either until his sister convinced him otherwise Masugu was even expecting Yui to not remember since it's really a thing everyone does
Even if the dude had slit the Inspector's throat she wouldn't have given two shits and would have left him to die right there. As long as her cases are successful, nothing will get in her way, even the mortal sacrifice of one of her own team members.