A 34-year-old man who threatened to kill passengers on a Greyhound bus has entered a guilty plea to one count of disorderly conduct.  Witnesses say Margarito Vargas-Rosas was apparently drunk last month when he threatened his fellow passengers.  A charge of making terrorist threats was dropped as a part of his plea agreement.  He was sentenced to 52 days in the Racine County Jail, but the judge gave him credit for time served.  A 9-1-1 call came from a passenger on the bus, but the driver thought deputies were chasing someone else.  A tire-deflation device was used to stop the bus a little over two miles past the state line into Illinois.