The Ancient Illyrians were great raiders (much like the Kretans of the same time), and thus rarely had anything resembling peace. They certainly had no compunction about sacking and enslaving their neighbors, and the Epirotes were among their victims (as mentioned, one famous raid enslaved the entire population of the Epirote capital). There also didn't exist any strong central authority; the inability of Queen Teuta to control her "pirates" was one of the reasons for the eventual Roman conquest of the Illyrians.