He tried to commit suicide at home due to his continuous abuse at the hands of bullies at school.
This suicide ended in failure, but he later met Sakata who answered him on an internet forum where he posted his intent to kill himself.
To get revenge on the bully, he received the “gift” from Sakata.
Herein he displayed his potential and became capable of killing people in just a couple of days.
Hurt by the idea that he used this ability to get revenge on those that did him harm, he realizes that there is considerable responsibility and emotional weight that comes with such great power.
Sakurai put himself in serious danger, going up against Izumi, who was massacring people — he believe that this was truly the right and just way to use his ability, his “gift.”
He first did this spontaneously when he saw those who were being attacked by Kappe Aliens in a mission after being revived by GANTZ.
Given his youth and age, it’s sometimes hard for him to judge successfully between what is right and what is wrong, but the passion he put into all of his actions bring vitality to those around him, as they see him so energized in whatever he does.
In this sense, his straightforward personality is actually his strongest weapon.
--- Oku-sensei’s comment:
Originally, I had been thinking of a different story to write outside of “GANTZ”, one involving super psychic abilities, and dragged a character over from that.
Maybe this affected things as he became the main character suddenly after finishing Kurono’s story arc.
His sort of shallow sense of justice is all the more pronounced, being that he’s a high school student.