The Crescents established a strong system of law and order very quickly after the Revolution ended, so the nation Ed grew up with was a peaceful, law-abiding one. While most people have an armament of some sort, actually using it is an extremely rare event. He's trained in basic marksmanship, but being attacked with a knife is an utterly new experience for him, and he won't soon forget it. He knows no self-defense techniques, has no experience de-escalating deadly fights, and to top it off, he barely knows this girl and has no clue what she's capable of. He genuinely thought he was going to die. If he had been next to that door, he would have been stabbed for sure.

Her lack of restraint is the reason he doesn't trust her very much. The thought that he could get stabbed on accident just by being in unexpected places at unexpected times frightens him. Technically, he should have gone to the authorities, who could have arranged for Sofie to get professional help as well as ensure his safety, but...well, she's attractive, and he doesn't exactly want her to leave. Male hormones are definitely playing a part in his decision-making. :P

Plus, he's still curious about her, and like he said, he's prepared to give romance another shot with her. He just has to be more wary from now on until he can work out a way to make her feel safe (and thus not feel the need to barge into rooms with a knife).

Making his lady feel secure is one of the foremost duties of a man, no? ;)