How could Lawrence III not know that his actions woud cause The End Of The World As We know It? He had that stone plate with the legend on it and he knew the legend. It's hard to not think he is evil instead of a total egomaniac.
Well, he could have regarded it as just legend, you know. Myths that aren't true.
He thinks that he is the chosen one mentioned in the legend and that capturing Lugia will be taming the beast of the sea, he probably assumed that once he caught Lugia the weather would stop but of course the legend really meant Ash taming the underwater current.