HoH as a complete one word sentence? Absolutely. The use of the null prefix leaves some ambiguity (which hopefully is made clear by either the context of the conversation or other words in the antecedentes). Among other things, the null prefix can be used to represent a third person singular subject doing something to a third person singular object, as in the example you give.
I'm not quite sure what you mean, but perhaps I can guess what is giving you problems. In English we first say who is doing the action (the subject) then the action itself (the verb) then who the action is done to (the object). In the sentence "Kahless killed Molor", Kahless is doing the action (the subject), killing is the action (the verb), and the action is done to Molor (the object). The basic English sentence order is Subject Verb Object (or SVO).
Many earth language put those same words in the same order (like Spanish, for instance). However, many languages actually use a different order. Subject Object Verb (SOV - that would be "Kahless Molor killed" if we were to change the English to match that) is actually the most common word order and is common in Latin and Japanese among many others. There are a couple Earth languages that put the words in the order Object Verb Subject (OVS - that would be "Molor killed Kahless" if we were to change the English to match that), but they are not well known.
Klingon is like those rare and little-known Earth languages that use OVS. Klingon speakers first say who the action is being done to (the object) then the action itself (the verb) then who is doing the action (the subject). In the sentence "molor HoH qeylIS", the action is done to Molor (the object), killing is the action (the verb), and Kahless is doing the action (the subject). So in English, we say that as, "Kahless killed Molor."