No, the 2009 action/crime/drama Rampage was not based on real events.
Rampage is a story written by Uwe Boll (born June 22, 1965 in Wermelskirchen, Germany), who also directed and produced the film.
Boll has a total of 35 titles to his producer credits, 26 to his director credits, 15 to his writer credits, and even 5 titles in which he acted.
The film starring Brendan Fletcher, Shaun Sipos, Michael Paré got an overall rating of 6.4 stars out of a possible 10 by over 12,000 votes on IMDb.
An interesting piece of trivia about Rampage is that the name of the character Evan Drince is an anagram, the letters can also spell "Can Never ID."
Also here's a tip that may help you enjoy one scene in the movie; watch closely in the bingo hall scene, the players that you see are not actors, their reactions are real when Brendan Fletcher takes and calls out a number.
In updated Uwe news, Rampage was not the last of character Bill Williamson, this year Uwe brought him back in Rampage: Capital Punishment.