Why did the giver apologize to Jonas after sharing the memories of war?
because it was a very sad memory for him, and it was filled with violence and bloodshed
The Giver is actually concerned about Jonas and the community. Since if Jonas drowns in the river, he will die, which will mean that the memories that Jonas has from The Giver will be released to the whole community and they will have to go through the same thing as the last time when Rosemary's memories were released to the community. The memories cause people in the community pain.
They plan to have Jonas escape. Jonas will ride away from the community and let the memories gradually go back into the community and the Giver will stay and help the people cope with the new memories. Ultimately Jonas takes Gabriel out of fear that they will kill the child (Gabriel doesn't sleep without Jonas' memories of warmth and settle rocking).
They believed another society existed because the Giver had memories of things that didn't exist in the community where he and Jonas lived. The Giver was able to share with Jonas memories of colors like red that weren't present in the community, and memories of love and family, which was a social construct that didn't exist in the community either.