The Collaborative International Dictionary of English v.0.48:

Justifiable \Jus"ti*fi`a*ble\, a. [Cf. F. justifiable. See
   Justify.]
   Capable of being justified, or shown to be just.
   [1913 Webster]

         Just are the ways of God,
         And justifiable to men.                  --Milton.

   Syn: Defensible; vindicable; warrantable; excusable;
        exculpable; authorizable. -- Jus"ti*fi`a*ble*ness, n.
        -- Jus"ti*fi`a*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

justifiable
    adj 1: capable of being justified

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "justifiable":
   admissible, allowable, balanced, cogent, condonable, credible,
   defensible, deserved, dispensable, due, equitable, even,
   evenhanded, excusable, exemptible, expiable, fair, fair and square,
   fit, forgivable, good, inoffensive, just, justified, lawful, legal,
   legitimate, legitimized, level, licit, logical, meet,
   meet and right, merited, pardonable, permissible, plausible,
   proper, rational, reasonable, remissible, right, right and proper,
   rightful, sanctionable, sane, sensible, sound, square, tenable,
   unobjectionable, venial, vindicable, warrantable, warranted,
   well-argued, well-founded, well-grounded, wholesome