Ms. Grundy's Ex-Husband
Obviously we have yet to meet this character on-screen (he might not even exist), but in terms of motive he has a very good reason to try and ruin the life of the boy who "stole" his ex-wife's heart. Grundy said that her husband was brutally violent towards her - so much so that she had to change her name and hide from him - so it's possible that he finally tracked her down and exacted his punishment.
The one flaw in this theory is that Grundy kisses a student before she sees him off, and yet that student is not killed by an ex-husband mad with jealousy. A possible alternative explanation is that Grundy was killed by the father of one of her victims, who found out about her nasty habit of preying on underage boys. However, that wouldn't explain why Fred Andrews was shot.
A Red Herring
Now, to the final and most likely explanation, given the nature of the show: the man under the black hood is nobody special - just a hit man hired to carry out the attack on Fred and the murder of Ms. Grundy. Based on Pop's description it seems like this is a man who has killed before, so he could merely be a professional doing a job. That opens up a whole world of possibilities as to who might have hired him. Hiram Lodge, perhaps, seeking to eliminate Fred as a threat to his marriage and business. The remaining Blossoms still have plenty of influence and connections, so maybe our killer was hired by one of them. He could be one of FP Jones' Southside Serpents contacts. He could have been hired by Alice or Hal Cooper (or both of them). If we assume that the man in the black hood is just a puppet with someone else pulling the strings, the possibilities are endless.
Who do you think is the ruthless killer stalking the people of Riverdale? Let us know your theories in the comments!