Right now I'm suspecting Ms. Grundy's ex-husband to be the one who's been going around attacking people. (In Season 1, Ms. Grundy told Archie that her ex-husband was very abusive towards her.)
Though I'm still holding on to the theory of Hiram Lodge (Veronica's dad) sending one of his thugs to go on this attacking spree. (Hiram and Hermione seem to be quite shady.)
I’m suspecting it was the serpent cause if you look at his eyes they’re exactly the same. Also it’s someone working close with jughead so you wouldn’t suspect
I've found this, and I can see the similarities too. So yea, it could be this older Serpent, but we'll see. I'm sure they'll eventually reveal the face behind the black mask as well as the person's identity.