Have developments in forensics and DNA technology reduced serial killings?

There aren't fewer cases, they just aren't reported as much. There was a big 'smack down' in the media after the Dahmer trial that we shouldn't glamorize these people or give them publicity. But people who are driven to kill will do so, no matter how good the detection methods get. There are some websites that peg known serial killings, and they make it clear they are everywhere all the time. Just do a bit of research, and you'll probably be shocked at what's going on out there.