Having intercourse with the concubine of the ruler was an act of rebellion.
When David returned to the palace, the ten concubines involved were sent away to live the rest of their lives in isolation (2 Samuel 20:3).
A concubine, whether purchased (Exodus 21:7-11; Leviticus 25:44-46) or won in battle (Numbers 31:18), was entitled to some legal protection (Exodus 21:7-12; Deuteronomy 21:10-14), but was her husband's property.
Concubine is second class wife; good for pleasure and the begetting of children but with no (or limited) inheritance rights.
The concubines death was thus in a sense natural justice; the manner of it perhaps poetic justice.
The Levites action in cutting up his concubine is extreme but certainly no less extreme than the injury done to her, to him and to the whole polity of Israel.