first five High Summoners? i thought there was only 4 including Braska before Yuna took out Sin.

Sin was meant to be a armour for Yu Yevon, made with massive amounts pyrafles and gravity spells.
when Yu Yevon died and lost control Sin went off on 3 directives it was programmed with. destroy all technology above a certain level. if too many people came together to reduce those numbers and to defend itself with lethal force on being attacked.

it also went for the sin spawn that dropped off its body as they were already large masses of pyraflys.

during the game events Jecht is able to effect sin from within, reducing the carnage.