I say were b/c they're both pushing 70 now and the beef long died but when they had beef it was because Chadrick came from a wealthy family who bought him into the police force back in theday- Mulligan did not respect Chadrick because he didnt earn it which is where the beef stems from- Mulligan grew to respect him quickly but Chadrick didnt want his respect so they remained enemies even tho Mulligan would try and bury the hatchet with him