Rem's encounter with Subaru at the end of episode 5
The short answer is... Yes, he was killed because of his witch smell.

Rem is very suspicious of Subaru in this time frame. He is a new person, entering a household she has grown to love. Now think about it: if a new person, Subaru, comes in, has distinct clothing that she's never seen before, is very 'friendly' with Emilia the main guest of the house, and smells like the witch. It's not looking good for our favourite little protagonist in her eyes.

Furthermore, we saw a scene somewhere in there where Roswaald whispered to Rem (and Ram I think) to keep an eye on him, as he was also suspicious, feeding her the seed of doubt about Subaru, even more so than before. Everyone was suspicious. He could have been a potential threat to their claim for the kingdom's throne.

Now we move onto Rem's personal reasoning.

This part is a spoiler for a later episode. If you don't want to know spoilers, don't read this part.


   As we know, Rem's family and her whole village, aside from herself and her sister, were killed by the witch's cultists, an organisation that, as stated, worships the Jealous Witch. Now think Subaru, a suspicious character who just appeared, smells just like the witch.





   We can tell from this that she has a personal grudge against him for this. In conclusion: Yes, you're correct. Rem killed Subaru because he smelled like the witch, which she had personal history with.


Edit 1:


   I believe the change in Rem around Subaru could be an inkling of sympathy for him. For example, when he wakes up, he is sporadic, screams and then realizes the twins are looking over him, at which point he is back to his 'playful, energized' self.

 I feel like she would see him as a 'madman' who isn't right in the head, to the extent that she is friendly with him, as to not convey her concern or trigger his madness. It could also be that she doesn't want to show disrespect to him, as he is a house guest, doing so would tarnish her own reputation, so in public, it is best to stay friendly.


Edit 2:

As the question states, 'Am I wrong?' about the dual personality of Rem. Personally, I believe you're right and wrong at the same time. I believe they're the same personality. It's just that Rem is putting on a brave face whenever she smiles. A quote I very much like, which I heard from someone, is "Don't trust someone just because the smile." I think that applies here.

Rem smiles a lot, due to the fact that she has to keep a brave face, as to not disappoint her sister, as well as to not tarnish the manor's professionalism.

Think of it, in terms of two girls.
Girl 1 (Rem) is weaker than Girl 2.
Girl 2 (Ram) was stronger, though lost her weapon, so she's now weak and can't use a weapon anymore.

If Girl 1 were to just sulk and throw her weapon away, when she can very well still use it, how would that make Girl 2 feel? This could also be why she keeps a smile on her face. She's that used to putting up her smile, to not disappoint her sister, that she does it with everyone. It's become a natural thing.

Now onto the subtitles on the last picture you included, as they reminded me of something else.

She said "My time has been stopped". As in, there was no way for her to move past that incident that she was stuck in her own madness, revenge stricken madness. Subaru comes along and saves her from this, later down the line, so that her time keeps moving (I'm not really sure how, I'd have to watch the episodes again).

However, this doesn't necessarily mean that the reason you provided was the right one. She stopped trying to kill him, as she realized, through talking to him, through spending time with him, that he was actually a trustworthy character, in the sense that he wasn't anything to do with the witch's cult.

He openly admitted to her that he smelled like the witch. He wasn't trying to hide the fact, he was also unsure of why it existed. Rem's a perceptive little demon, so she picked up on how his emotions and expressions were in the end: trustworthy.

However, having only just met him in episodes 5/6, he was just another witch's disciple, a threat she had to eliminate for her own gain, for the revenge of her family, her sister and herself.