What's Cassian supposed to do - physically restrain her? If she's not going to listen to his arguments about why we should just let them be, why would he imagine that she won't just go right ahead and release them as soon as he lets her go?

Regarding the point about not killing people who've surrendered, Cassian and Winifred are in agreement, which is why he's never advocated killing them.

For Cassian's part, he's not concerned with appearing "professional" because he's not looking for work and he's not afraid of what any officials might think. He also doesn't believe Lamm or his crooks have any friends that would stand up for them once they've been taken down and he believes his station as the son of (low-ranking and foreign) nobility will protect him.

As for the crooks seeking revenge, Cassian doesn't for a second believe that they won't do that just because we let them go now. These are evil men with no morals - they'll perceive Winifred's actions as a sign of weakness, not decency.

The last point doesn't work for him at all. People that need killing deserve that, no matter what theological consequences it may have.

Anyway, strictly OOC, I hope Winifred isn't going to be like this all the time. If she continually acts against Cassian's perceived better judgement and does so to his face, he's not going to see any point in continuing to associate with her. As things stand, she's just a bossy serving girl - no one who's opinion he has to care about.