He wanted to give Bruce a wedding gift. The gift being to live one day in a world where The Waynes never died, so he went back in time and stop the murder.
The resulting chain of events made Hal Jordan kill himself after infected by Joker, Selina in Arkham as she thinks she's an actual cat, Dick Grayson became a murderous Batman, and Booster himself lost his mind as he keeps trying to fix things but keeps failing
"My name is Wally West. I'm the fastest man alive!"
I'll try being nicer if you try being smarter.
Truly Booster's finest hour...