The hand doesn't seem to be Hunter's, but at the same time the hand and choice of clothing do not seem to be Drew nor Brenna either, the only two other people who might be involved here.
I don't think that's young man hunter either, the hand still seems bigger compared to the one in his previous teasers, maybe Kevin? But Kevin is now Buff Kevin, and he, like, is literally the LEAST involved character with this pairing, and it still wouldn't explain the clothes.
OK here's my theories.
A) The Mysterious Veterinarian/Doctor Partner of Hunter, the one who provides him the drugs, here to recover the various canisters he lent Hunter and finding Sterling in the shed, thus needing to put him down to avoid being discovered. If we ascribe to the "Hunter's partner is the Nurse Rachida's Husband because he mentioned him when talking about Starling's Vaginoplasty in the Force Fem Bad Ending" it would also explain the choice of clothing, and he would also have a further reason to want Sterling's dead since he is probably a subscriber of Hunter's stream and watched Sterling fuck his wife while affected by the drugs he had provided Hunter.
B) Same as option A but Eileen is actively trying to get a hold of the drugs herself, and this is all a ploy to get him to show her where they are. Why? Maybe she's working with the doctor, maybe she wants to start her own meth empire on a RV in the Nevada desert and she needs the goods, maybe she wants to drug the whole family in retaliation for what Brenna did to her and then out them as incest freaks, anyway essentially the same as Option A but she is not a casualty in this and she's actively engineering the situation.
C) Uh... Sean maybe?