Dragon Ball Z fucked up by letting Goku kill Broly. The only reason it happened was so it would appeal to kids and give the message of "never giving up". Dude had a 1.4 BILLION power level to Goku's SSJ 30,000+. Broly fucked up TWO Full Power Super Saiyans, two Ascended Super Saiyans and a Super Namek (I know you reading this Piccollo, I'm sorry) but they pulled some stupid bullshit to kill him.
Plus, Broly's power level increased dramatically during every fight so imagine if he had stayed around and killed the everyone on Earth.