Timmy really didn't mean to do what he did, but at least the undo button brought everything back to normal.
Except in real life there is no undo.
Everything that the gun deleted was not quite the same when brought back. The girl he deleted and brought back, he would never be able to have children now, and she would live in agonizing muscle and bone pain from that point on. Same with those who had body parts deleted - they would live with pain and suffering in those parts, one of them would get breast cancer because of him, and lose both breasts for real.
Timmy was just a little boy, and accidents happen. But sometimes a thing can never really be completely undone, not like it never happened - that was a fantasy, even those who believed in and practiced magic would tell you that there is always a price.
Timmy should have learned his lesson, but he was careless again, and the last thing he experienced was the gun he carelessly dropped hitting the ground and being triggered from the impact, and finally his head being delet-
Agony. Sheer agony. Pain shooting through his head. Timmy had never felt pain like this before. Through pain, Timmy could one of the girls - she had found the gun and figured out what had happened. She was trying to help, realizing that a headless boy wasn't normal. She had hit the undo button and made Timmy right again.
Except for the excruciating pain. Timmy could barely think, and he heard a voice saying, "please delete me" before he even realized it was him. All he wanted to do was stop the pain. The girl looked scared, then ran away with the gun still in hand. There would be no more deleting for Timmy, he would have to learn to live with the pain.