I'm probably missing your point, but

  public static IList<Ephraimite> FindEphraimitesToKill(IList<Ephraimite> ephraimites)
  {
      var ephraimitesToKill = new List<Ephraimite>();
      foreach (var ephraimite in ephraimites)         
          if (ephraimite.Speak("shibboleth") == "sibboleth")            
              ephraimitesToKill.Add(ephraimite);                    
      return ephraimitesToKill;
  }

or:

  public static IList<Ephraimite> FindEphraimitesToKill(IList<Ephraimite> ephraimites)
  {
      return ephraimites.Where(e => e.Speak("shibboleth") == "sibboleth").ToList()
  }

is not that verbose.