Psycho killers and maniacs used to be, but now play games instead? Seriously?
That is like claiming people stopped doing lewd stuff, but now play lewd games instead.
Admittedly, there is a rise and fall in statistics with serial killers, but I would not attribute that to games or lack thereof. The advancement in forensics allowed to identify murders as being serial murders in the first place. Not everything is as clear cut as Jack the Ripper cases. But that same advancements allowed to catch murderers before they became serial. Or even deter them or drive them to use methods that are again not detectable as serial cases.
If the gov would be really interested in effectifly reducing stuff like killing sprees by banning or not banning stuff, they should ban the news. No other thing inspires wannabe killers more.
If you want to hear a plausible idea, it may be due to leaded gasoline. Lead poisoning increases aggression. Leaded gasoline was introduced in the 1920s and banned around the 1990s.