goku didn't really kill broly in the first movie, somehow broly survived and the 2nd film shows him escaping the planet in a pod and crashing in a snowy area on earth and becoming frozen in a glacier, then years later goten and trunks go to a village near it in search of a dragon ball, broly awakens, beats the crap out of them and super saiyan gohan, shenron makes goku appear from otherworld temporarily and goku, gohan, and goten use a triple father son kamehameha on broly sending him into the sun, killing him for good. third movie is not really broly but some bio creature made with his dna.
Ah. I remembered the first wrong then. It sounds like, if Broly's alive, they must be discounting the two that came after. I'm not surprised they went back to him. I remember the first Broly movie being fairly popular back when I paid attention to Dragonball related happenings.