Unkind is defined throughout "The Spanish Tragedy" as "unnaturally," "brutally," and "cruel."

"O unkind father! O deceitful world! With what excuses canst thou show thyself? With what devices seek thyself to save from this dishonor and the hate of men, thus to neglect the loss and life of him" (IV, i, 7-11).

Bel-Imperia is asking Hieronimo what justification he will use to avenge Horatio's murder. She calls him an "unkind father" because his plan to murder Lorenzo and Balthazar is an unloving act towards Horatio. Bel-Imperia believes that murder would not be the right form of action.

"Accursed brother, unkind murderer, why bends thou thus thy mind to martyr me?" (III, ix, 5-6)

Bel-Imperia is wondering why her brother, Lorenzo, murdered Horatio. She does not understand his cruel and unnatural act. She is hurt that Lorenzo betrayed her in killing the man she loved and wonders why? Because of jealousy? Because of power?