what important decision did johny make? why did he make it?

The decision was not to see his mother. He didn't want to see her because he didn't want to be hurt by the way she treated him. Text evidence below;

A nurse appeared in the doorway. "Johnny," she said quietly, "your mother's here to see you."

Johnny opened his eyes. At first they were wide with surprise, then they darkened. "I don't want to see her," he said firmly.

"She's your mother."

"I said I don't want to see her." His voice was rising. "She's probably come to tell me about all the trouble I'm causing her and about how glad her and the old man'll be when I'm dead. Well, tell her to leave me alone. For once" ---his voice broke--- "for once just to leave me alone." He was struggling to sit up, but he

suddenly gasped, went whiter than the pillowcase, and passed out cold."