Mobile hunter-gatherer ethnographies (the most primitive we had to study), are awash with depictions of organized, lethal violence against fellow humans, or active elimination of competition. Applies to Eskimos, Fuegians, Aborigines etc. Cruelty to animals and not always using all parts of the prey are among other peaceful savage mythbusters anyone access to any Anthropology library can check for herself.
With regards to living to old age, it most definitely is a survival advantage to offspring. Even in today’s affluent societies, families with small children heavily rely on grandparents for all-round help, and the assitance doesn’t diminish even later (my grandmother paid for my education, for instance). In pre-modern, subsistence societies having grandparents must’ve been a great boon to a person all the way to reproduction age.
There’s also the fact that such people are an important repository of knowledge in a pre-literate culture. A good example I read of once was an old lady who among other things knew of a root that her tribe ate during a drought when she was a girl; it tasted bad and wasn’t very nutritious, and in the intervening time had been forgotten because no one dug it up when there were other things to eat. Forgotten by everyone younger than her, and if another drought came she could have told them about that root - whereas if people never lived passed 25, no one would know. People living longer means that knowledge can survive better. An elderly human can contribute a lot more to his/her grandchildren than an elderly deer can, simply because information is so much more important to humans.
As it happens we live ( IIRC ) about three times as long as creatures with our metabolic rate normally do. We’ve already been pretty heavily selected for increased lifespan apparently.