Jonas' plan had been to escape from the community within two weeks, during the yearly Ceremony. He and the Giver had been planning everything carefully. The Giver would give him memories of courage and strength so that he would be able to cope with the challenges he would face during and after his escape.
The Giver would also save food from his own meals for Jonas to take with him so that he would have enough sustenance for two weeks. He would request a vehicle and hide Jonas in its storage area. He would then drive him out of the community. A search would obviously be done and Jonas' bicycle and clothing would later be found next to the river. The Giver would then report that he had been lost in the river.
The plan seemed a good one and would work. However, Jonas made the decision not to wait when his father said, at the evening meal, that Gabriel would be released first thing the next morning. Jonas had learned that release meant that Gabriel would be killed. The Giver had shown him a recording which depicted how a smaller twin had been released by his father. Jonas was shocked when he saw his father kill the baby by lethal injection and then dispose of the body.
When his father mentioned that Gabriel, the problematic newborn they had been taking care of, had had his chance and that his time was up, Jonas decided to rescue him. Jonas loved Gabriel and had been taking special care of him by giving the newborn pleasant memories, which calmed him down and eased his sleep.
Jonas took leftover food from the neighbors' doorsteps before it was collected and decided to escape by stealing his father's bicycle, which had a child's seat at the back on which Gabriel could be accommodated.
Jonas took a tremendous risk, for he would be killed if found. He decided to forgo receiving memories of courage and strength, but it is obvious that his love for Gabriel gave him an abundance of both. In the end, he would save not only Gabriel and himself but, to a certain extent, the community as well.
Near the end of the book, Jonas and the Giver create a plan for Jonas to escape and fake his own death. This will allow him to release his stored memories to the Community, and find a new life outside. However, he changes the plan when he discovers that his baby stepbrother Gabriel is going to be euthanized; Jonas steals Gabriel and leaves without learning the last important memories.
There had been no time to receive the memories he and The Giver had counted on, of strength and courage. So he relied on what he had, and hoped it would be enough.
(Lowry, The Giver, Google Books)
Jonas had previously thought mostly of himself; he was trying to escape not only for the good of the Community (although he cannot be certain that the memories will return), but so he can witness and experience different places and things. Seeing that the Community is still going to continue its practices of euthanization, he becomes angry and determined to save Gabriel; this decision is impulsive, driven by emotion. For Jonas, it is more of a symbolic gesture of defiance; if he and Gabriel die outside the Community, it will be on their own terms. However, he is also hoping to find other people and give Gabriel the chance of a new life, one without the Community forcing every member into an assigned role.