I'm not a native but did learn to speak fairly fluently while living in the Philippines for a few years and would be happy to help. To be honest both sentenced sound very incorrect. A lot of it has to do with what you're focusing on in the sentance. As a native English speaker myself I understand it feels more natural and easy to use all the (um, mag) actor focus verbs (I made this mistake as well at first) but it's much more natural in Tagalog to speak using object focused verbs (in, an,) as it let's the listener know what is being talked about more clearly. Kumain siya niya. Is very vague and sounds more like a command and what niya means is not clear. Sounds more like HE ate... something but we don't know what since pronouns are more loose in Tagalog then in English. say kinain niya ang (insert object). Then it's know exactly what was acted upon. Pumatay (even though it sounds like to kill to us it doesn't mean that) is used only if it just happened within the last few minutes. Kinda like saying 'he was just killed' or 'just died' If you want to say somebody died awhile ago or I will kill you. Use the object or location focused verbs (in) papatayin kita (i will kill you) pinatay siya (he was killed or he died depending on context) I hope this helped.

Kumain and pumatay is the people in the sentence are the ones who did the action. Kinain and pinatay means the subjects in the sentence are the ones to whom the action is done to .

Or wait...maybe what you mean is...it is okay to say KUMAIN NA SIYA but not okay to say PUMATAY NA SIYA. Kumain na siya is, He already has eaten. Or he has finished his meal. Pumatay na siya is, He has already experienced killing somebody.

Your phrases both sound confusing and I couldn't understand what you mean by any of them. Anyway, these are the proper ways of saying whatever you might want to say... PInatay niya ito. He killed her, him, it. PUmatay siya. He killed somebody. Pinatay niya si (insert person). He killed (insert person). Pumatay siya ng ( animal or a non-person). He killed an ( animal or a non-human being). Pinatay siya ni (the kiiler). He was killed by (the killer). Pinatay is always followed by niya and pumatay is always folllowed by siya. Pinatay is what or who has been killed. Pumatay is the one who killed. niya followed by siya or siya followed by niya doesn't make sense. Kinain niya siya doesn't make any sense too. Kumain siya ng ( something)for He ate (insert the food or what has been eaten). Kinain niya ang ( the food). He ate the ( insert the food). If you use Kinain niya siya...It would mean He ate her or him, because siya refers to a human being. Kinain siya niya would also mean, He was eaten by him, or by her.

As Wayne said, both of those phrases are incorrect. Depending on what you meant to say, these would be the correct ways to put them: PINATAY siya niya = PINATAY niya siya = “A” killed “B” (“A” is “niya”, which could be he, she , or it. “B” is “siya”, which could be him, her, or it). KINAIN siya niya = KINAIN niya siya = A ate B (Same substitutions as above apply for A and B, although I hope cannibalism isn’t the topic) Pumatay siya NIYAN = “A” killed one like that. (This time, “A” is “siya”. “Niyan” is “that”, an object nearby.) Kumain siya NIYAN = “A” ate one like that/some of that. Also, depending on tone and context, the last 2 examples that used “niyan” may mean, “It seems that he/she/it did the act of killing”; “Apparently, he/she/it has eaten”. “Niyan” in such cases takes the meaning of “by the looks of it”.