Justifiably

from WordNet (r) 3.0 (2006)

justifiably
    adv 1: with good reason; "he is justifiably bitter" [ant:
           {inexcusably}, {unjustifiably}]

from 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]