The Collaborative International Dictionary of English v.0.48:

Imbitter \Im*bit"ter\, v. t. [imp. & p. p. Imbittered; p. pr.
   & vb. n. Imbittering.] [Pref. im- in + bitter. Cf.
   Embitter.] [Written also embitter.]
   To make bitter; hence, to make distressing or more
   distressing; to make sad, morose, sour, or malignant.
   [1913 Webster]

         Is there anything that more imbitters the enjoyment of
         this life than shame?                    --South.
   [1913 Webster]

         Imbittered against each other by former contests.
                                                  --Bancroft.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Embitter \Em*bit"ter\, v. t.
   To make bitter or sad. See Imbitter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

embitter
    v 1: cause to be bitter or resentful; "These injustices
         embittered her even more" [syn: embitter, envenom,
         acerbate]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "embitter":
   acerbate, afflict, agent provocateur, aggravate, aggrieve,
   alienate, amplify, anguish, annoy, antagonize, arouse, augment,
   bitter, break down, bring to tears, bristle, build up, chafe,
   crush, cut up, damage, deepen, desolate, deteriorate, dilapidate,
   draw tears, endamage, enhance, enlarge, envenom, estrange,
   exacerbate, exasperate, fret, grieve, harm, heat up, heighten,
   hot up, huff, hurt, impair, incense, increase, inflame, infuriate,
   injure, intensify, inundate, irritate, madden, magnify, make acute,
   make worse, miff, nettle, oppress, overwhelm, peeve, pique,
   prostrate, provoke, put back, rankle, rile, roil, ruffle,
   set against, set at odds, set up, sharpen, sorrow, sour,
   stir the blood, stir up, torment, vex, weaken, work up, worsen