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”.