Jonas steals his father’s bicycle because it has a child seat on the back, and Jonas takes Gabriel with him to prevent his release.
As Jonas is leaving the community for Elsewhere, he makes three transgressions that would result in Release if he was caught. He is out at night, he takes food, and he steals his father’s bicycle.
Jonas does not hesitate because he knows he will be killed, or because he feels bad for his father. He hesitates because of “not wanting anything of his father's” after he saw him kill the newborn baby. Jonas is also not sure the bike will fit him.
But it was necessary because it had the child seat attached to the back. And he had taken Gabriel, too. (ch 21, p. 166)
At this point, there is no going back. Jonas knows that he will not survive if he is caught. Both Jonas and Gabe would be killed. So breaking rules is not an issue. He cannot stay, now that he knows the truth about his community.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book) (p. 166). Houghton Mifflin Harcourt. Kindle Edition.