Probable spoilers ahead:
In response to your comment; IKR?! He's so scary at that part.
I'm lowkey starting to think that Walter has this "psychopathic" traits (probably him or his alter ego if he really has DID) and I SUSPECT him linked to Mark's brother's death. From Mark's episode, I think that I can confirm that the stalker is Herbert trying to see if Walter would remember anything regarding the incident (since Mark himself stated that the stalker resembled Walter, which is weird since Walter himself was in school when this stalking incident happened).
Additionally, the stalker was trying to convince Mark to avenge his brother's death by saying that the person arriving at the station is the cause of Alex's death (which led me to believe more that this is Herbert since he's Mark's brother's bestfriend).
AAAND, during Waterfront scene with Tora in Parker's route, you will see Walter memory's flashing back where Walter is talking to "Walter" saying that "when are you going to stop lying to yourself when you keep on denying your identity". For this part it got me thinking that Herbert is Walter, and Walter is Herbert so I'm pretty unsure about this. Not to mention Lars stated before that as twins, both Herbert and Walter would try to swap to confuse them, and Lars said it was pretty hard to distinguish who was who compared to the Thornton brothers (This can be found on Lars' route).
There's like so much theory going on with me as to why Walter is SUS and scary at times.
Oh and also, to top it up, Phylice and Edgar basically said Herbert is mostly locking himself in the room which leads me to believe that he is sneaking out to do those stuff.