Who were and who are the Kenites (Hebrew word meaning children of Cain, the first murderer)?
Names are not often unique to a single individual. There can be more than one Cain in world history. Thus, at best you can say that the Kenites trace their origins to a patriarch named Cain, but not necessarily the same Cain who killed his brother Abel; in fact, that would not be a possibility since only Noah (a descendant of Seth) and his family survived the flood. For example, one of the towns listed among Judah's inheritance was called Cain or Kain, depending on the translation you use (Joshua 15:57).
In Judges 4:11, a literal translation of the verse says, "Now Heber the Kenite separated from Kain from the descendants of Hobab the father-in-law of Moses ..." From this, most conclude that the Kenites were the people of Kain.
The Kenites are first mentioned in Genesis 15:19 as one of ten nations occupying the land of Canaan (Genesis 15:19).
Moses' father-in-law, Jethro, who was a priest in Midian, was actually a member to the Kenites (Exodus 18:1; Judges 1:16). He was also called Hobab (Numbers 10:29). Some of them traveled with the Israelites, living in the Wilderness of Judah, indicating they were nomadic people.
Balaam, in a prophecy, mentions Edom, Amalek and the Kenites together, giving the idea that these nations lived near each other or with each other (Numbers 24:18-22).
It was Heber, a descendant of Hobab (or Jethro), whose wife helped Deborah in the defeat of Sisera (Judges 4:11,17).
Because of this close and friendly association between the Kenites and Israel, when Saul was preparing to attack the Amalekites, he warned the Kenites to move out before he struck (I Samuel 15:6). This again indicates that the Kenites were nomads and lived among the nations on the south edge of Israel (I Samuel 27:10). But they did have cities of their own (I Samuel 30:29).
From the Kenites, several sub-families arose: "And the families of the scribes which dwelt at Jabez; the Tirathites, the Shimeathites, and Suchathites. These are the Kenites that came of Hemath, the father of the house of Rechab" (I Chronicles 2:55).