WordNet (r) 3.0 (2006):

do in
    v 1: get rid of (someone who may be a threat) by killing; "The
         mafia liquidated the informer"; "the double agent was
         neutralized" [syn: neutralize, neutralise, liquidate,
         waste, knock off, do in]

Moby Thesaurus II by Grady Ward, 1.0:

148 Moby Thesaurus words for "do in":
   KO, assassinate, bankrupt, beat, beat all hollow, beat hollow,
   beguile of, best, bilk, blast, blot out, bump off, bunco, burn,
   burn out, cheat, chisel, chouse, chouse out of, cog, cog the dice,
   con, cook, cool, cozen, crib, croak, debilitate, defeat, defraud,
   destroy, diddle, dilapidate, dish, do for, do out of, do up, drub,
   dust, enervate, erase, euchre, exhaust, fag, fag out, fatigue,
   finagle, fix, flag, flam, fleece, flimflam, fob, frazzle, fudge,
   get, give the business, gouge, gull, gun down, gyp, harass, have,
   hide, hit, hocus, hocus-pocus, hors de combat, ice, jade,
   knock out, knock up, lambaste, lather, lay out, lick, mulct, off,
   outclass, outdo, outfight, outgeneral, outmaneuver, outpoint,
   outrun, outsail, outshine, overfatigue, overstrain, overtire,
   overweary, pack the deal, pigeon, polish off, poop, poop out,
   practice fraud upon, prostrate, put, rook, rub out, ruin, scam,
   screw, scuttle, sell gold bricks, settle, shave, shipwreck,
   shoot down, shortchange, sink, skin, skin alive, stack the cards,
   stick, sting, swindle, take a dive, take care of, thimblerig,
   thrash, throw a fight, tire, tire out, tire to death, torpedo,
   trim, triumph over, trounce, tucker, undo, use up, victimize,
   waste, weaken, wear, wear down, wear on, wear out, weary, whip,
   wilt, wind, wipe out, worst, wreck, zap