Why does Rem kill Subaru at the end of Ep5?

Thing happened from minute 22
At the end of Episode 5, at night, Subaru is sick. He goes out to finding help. He is attacked. At 23:35 to 23:40, we hear the sound of iron weapon (which likely to be Rem morningstar). And Subaru is killed with brute force.

However, it is unclear why Rem do that. Because the relationship of Rem and Subaru is looking good. They both going to the village and have some talk. Rem also look happy then (at 17:29).

I did watch Episode 6, Subaru is very distance from Rem, and he is suspicious, along with witch scent that gets him killed. But does Rem kill Subaru in Episode 5 for same reason ?

Edited: From @ObviouslyJake answer, I would like to add some more detail Comparing from the time frame in Episode 5 and Episode 6
Similar: Subaru's witch smell (which is the main reason, according to @ObviouslyJake) Different: the relationship with Rem Episode 5: They talk, go downtown together. Rem is happy. (at I said above, with picture). In this episode, I can feel that Rem and Subaru become friends. Episode 6: Rem tries to be distance from Subaru. Also, Subaru fall in her garden twice, which she has to fix it. In this Episode, Rem and Subaru are strangers.
Is the difference in the relationship between Subaru & Rem contribute to Subaru death at the end of Episode 5, comparing to Episode 6. Or the Witch Smell is the only reason ?

Because there is two Episode, with some different choose, difference path, I think it must be something different here. I feel it have to be, but I can't find it.

Edit (Aug 31,2016):
Thanks to @Davies comment and @ObviouslyJake, thing I have in my head so far
In Episode 6, it is worst than Episode 5. Here we talk why Rem kill Subaru in Ep 5, despite good relationship.
My theory is that Rem has dual personality, Rem the maid, and Rem the Oni who hate Witch.

In Ep 5 Subaru has good relationship with Rem the maid. Result of being good relationship with Rem the maid:

• Rem smile at 17:29 (picture above)
  
• Rem gets Emilia teaching Subaru in the evening. (Episode 5, 17:57)
  
  

However, Rem the Oni still hate Witch, which lead to Rem kill Subaru because of Witch smell

Therefore, here is summary:
Rem will trying to kill Subaru because of Witch Smell unless the following condition is satisfied:

• Rem is dead (nothing to say, I suppose...)

or

• Rem the Oni is freed from the tragic of her childhood.
  

(spoiler, don't look if you have not watch the latest episode)

The second condition is satisfied when Subaru save Rem from the Demon beast in forest at Episode 11.
The proof is in Episode 18, from 20:00 to 21:00, Rem confirms that.

That is what I think, please tell me if I wrong.