Nick and Boris held game nights every weekend.  It was a ritual they had kept up for many years.  Boris had a tendency to win games often.  That night, while playing for money, Boris had once again won the jack pot and Nick was tired of it.  So when they came up with the idea for the big scare, he decided to get revenge for all of his losses... by replacing the blank bullets with the real bullets and thus  Boris was killed.  That's what you'd call a bad sport!