During episode 7, Beatrice was contracted to protect Subaru. When she is with him on the cliff she suggests he "die where she can't see him" so she won't "have bad dreams," about it. Maybe she was being sarcastic, referring to the fact that he should die, suggesting Rem's death was his fault? I'm not sure if it was sarcastic, but it seemed to me like she was suggesting he kill himself to Return By Death? It sounded like she was conveying that he wasted his chance to explain himself to Ram, this time, but could kill himself and try again? Ah, I don't know... She says "I'll help you escape this domain"... What was she foreshadowing here? Does she know?
It is because she is somewhat attracted to him (not in romantic means of course). Beatrice to some degree cares about him, otherwise, she would not agree to be in contract with him. That is why she said, "I you want to die, die at a place I cannot see. I don't want to have nightmare." Implicitly she is saying, "Don't let me see you die. I care about you, so if you die in front of me, I'll have nightmare about it."
That is why, rather than knowing about Return by Death, it is more like she cares about him and don't want him to die.
I think you're reading it the wrong way. The situation they're in has two main components to it:
Ram will stop at nothing to kill Subaru.
This is as she blames Subaru for Rem's death. She doesn't let Subaru touch Rem, "Don't touch her! Don't touch my little sister!" She then later ignores why Beatrice is protecting Subaru and tries to attack him justifying it with what happened to Rem: "None of that even matters! Get out of the way. Let me through. I must avenge Rem. If you know anything, tell me. Help me. Help Rem!"
Beatrice will protect Subaru with everything she has, but will only do it when he's near the mansion.
Quoting the scene you're on about:
Subaru: Why did you come for me? I...
Beatrice: The contract I entered into was to protect you.
Subaru: I thought you were only supposed to be my bodyguard until this morning.
Beatrice: You must be mistaken, I suppose. I don't remember discussing a time limit. Clinging to hope only serves your own convenience, I suppose. What was lost can't be reclaimed. You'll have no more chances to explain yourself to the older sister. You threw that away. No matter which is lost, those sisters won't ever be complete again, I suppose.
This shows, even though Rem hates his guts, and Beatrice knows, she will protect Subaru. And protecting Subaru is the reason that she went to the cliff.
We don't know the exact reason that she's doing this, but we can guess it's because she cares somewhat about Subaru.
Putting these two together, rather than saying "At the least, you need to die where I can't see you, or I'll have bad dreams, I suppose." She probably means 'please don't die in front of me, I'll suffer mentally due to it.' And as she wants to at least protect him somewhat, she offered to "help you escape this [place]."
However if she knew about 'Return by Death' it'd not make sense for her to go out of her way to kill him. She could instead just let Ram kill him.
She was confused when Subaru seemed to show little fear or panic when hearing about his impending demise by the Wolgarms' curses. She really does seem to let on that she knows at times. But I am confident she does not. When she sent him away after the unseen hand crushed Emilia's heart, she knew that Puck would soon go on a rampage and end his life.