It is most likely that Ay was the culprit, not Horemheb.

Ay became pharaoh directly after Tutankhamun, and forced Ankhesenamun into marriage with him. Had it been Horemheb, wouldn't he have been the "servant" whom the Queen spoke of in her letter to the Hittites? No, because Ankhesenamun never married Horemheb, but Ay. Subsequently, the Queen disappears from the records.

From this it is deduced that Ay murdered Ankhesenamun after he stole the throne from Tutankhamun, for he had only used her to "legally" become king. An old man, Ay died only a few years later, and Horemheb took the throne.

Many also believe Aye and Horemheb were in the murder together.

I think the murderer (if it was really a murder) was Horemheb. After all, he wasn't of noble origin while Ay was related to the royal family (as Tiye's borther and probably Nefertiti's father). So when Ankhesenamen spoke about a "servant", it is more likely that she was referring to Horemheb than to her uncle / grandfather. Maybe after Zannanza was killed, Ay tried to save the throne for Ankhesenamen and himself from Horemheb, and this can be the cause of Ay's name on monuments being changed to that of Horemheb.
There is no proof that Ankhesenamen actually married Ay. That ring with their cartouches can refer to a co-regency as well.

However, the other theory, that Horemheb and Ay both were responsible for Tut's death, also makes sense. It's not hard to imagine them agreeing that Ay will rule and after his death (which was near, for he was already old) Horemheb will rule. Maybe this was the reason of Akhesenamen feeling lonely and desperate and writing the letter to the Hittites.
To prove this theory I think it would be needed to know when did Horemheb marry Mutnodjmet. If it was about the time of Tut's death, then it is likely that this theory is correct.