In addition to bib's excellent suggestion vindictive, you might consider rancorous, which Merriam-Webster's Eleventh Collegiate Dictionary (2003) defines thus:

rancorous adj. (ca. 1570) marked by rancor [defined by MW as "bitter, deep-seated ill will"] : deeply malevolent {rancorous envy}

S.I. Hayakawa, Choose the Right Word (1968) distinguishes between vindictive and rancorous as follows:

Vindictive means spitefully vengeful, and suggests the harboring of grudges for imagined wrongs until the vindictive person, with satisfaction and perhaps even enjoyment, sees the object of his hatred suffer. ... Rancorous suggests a festering ill will, perhaps stemming from resentment, over some real or fancied wrong. It does not, however, like vindictive and spiteful, necessarily imply a desire to hurt—only a deep-seated malice.

It's interesting that, in Hayakawa's telling, vindictive, rancorous, and unforgiving span a wide range of possibilities as to the legitimacy of the grudge, from "imagined wrong" in the case of vindictive to "real or fancied wrong" in the case of rancorous to actual injury in the case of unforgiving (since forgiveness doesn't make sense in situations where there is no wrong to forgive).

In addition to bib's excellent suggestion vindictive, you might consider rancorous, which Merriam-Webster's Eleventh Collegiate Dictionary (2003) defines thus:

rancorous adj. (ca. 1570) marked by rancor [defined by MW as "bitter, deep-seated ill will"] : deeply malevolent {rancorous envy}

S.I. Hayakawa, Choose the Right Word (1968) distinguishes between vindictive and rancorous as follows:

Vindictive means spitefully vengeful, and suggests the harboring of grudges for imagined wrongs until the vindictive person, with satisfaction and perhaps even enjoyment, sees the object of his hatred suffer. ... Rancorous suggests a festering ill will, perhaps stemming from resentment, over some real or fancied wrong. It does not, however, like vindictive and spiteful, necessarily imply a desire to hurt—only a deep-seated malice.

It's interesting that, in Hayakawa's telling, vindictive, rancorous, and unforgiving span a wide range of possibilities as to the legitimacy of the grudge, from "imagined wrong" in the case of vindictive to "real or fancied wrong" in the case of rancorous to actual injury in the case of unforgiving (since forgiveness doesn't make sense in situations where there is no wrong to forgive).