If you are looking for injustice then "Baron o'Brackley" is pretty hard to beat although some notes suggest that it was a revenge/retaliation killing.
One very moving ballad is "The Border Widow's Lament" where the second verse
"There came a knight by middle day,
He spied his sport and went his way,
He brought the King that very night,
And slew my man before my sight"

suggests a totally motiveless killing. Possibly there was more to the original ballad than the story that survives?