The Metsudath David commentary on Tehillim, chapter 69, verse 5, seems to imply that Sinat Chinam is hating a person who never harmed you.
Netziv’s introduction to his commentary on Bereishit [HaEmek Davar] teaches that sinat chinam is hating someone because he belongs to a different group or because he has a different point of view or because he does things differently or a different custom.