According to Merriam Webster dictionaries, enmity means:
positive, active, and typically mutual hatred or ill will
How can "hatred" be "positive"?
“Positive” doesn’t always mean “good”, which is what you might be thinking. See M-W’s definitions for “positive”- https://www.merriam-webster.com/dictionary/positive - only definitions 6 and 8 (particularly 8) seem to have anything to do with “goodness”. The meaning of positive here I think is what the dictionary has under definition 2c:
c(1) : INCONTESTABLE
(2) : UNQUALIFIED
a positive disgrace
“Positive hatred” means that there is no doubt about the hatred’s existence (2c1), and/or that there is nothing but hatred (2c2).