How did halloween start  

The holiday origin is in an old Pagan festival related to the Irish and it is was called "Samhain". In this holiday they celebrated the end of the harvest season in he Gaelic culture. They believed that  on the 31 of October the boundaries between the world of the living and the dead overlapped. Now you can imagine whats the reason for the scary holiday costumes.

For more check the detailed Wiki article. 

Roughly 2,000 years ago, a people known as the Celts lived in the area now divided among three modern-day countries: the United Kingdom, Northern France, and Ireland. The Celts began their new year on November 1, meaning October 31 was their New Year's Eve. According the History Channel, "the boundary between the worlds of the living and the dead became blurred" that night.The Haunted History of Halloween (History Channel) (A&E DVD Archives).  This is an interesting and informative documentary that gets you in the spirit of Halloween. A 'must have' for those that love this holiday!You see how HALLOWEEN began, why we carve pumpkins, why children wear costumes and trick-or-treat, how ghost stories started, etc.