Kindness is a poor word choice, except in its most literal sense. Care is much more apt. Mammals' key evolutionary advantage over reptiles is that they care for each other, forming social groups which enable greater fitness. Likewise with primates > mammals, and humans > apes.
(One could argue that intelligence and brain size have more to do with evolutionary fitness, but I would speculate that intelligence is a result of increased care, not the other way around. Give a thing more time, space, and resources to grow, and it will.)
As a species, the more we collectively care for each other--through actions, not feelings--, the greater chance we have for survival and growth.
We are 'responsible' for one another, but in far more than just 'caring' ways. We take out the trash, we try to get along, we do all sorts of things every day that really fall into the category of 'acting responsibly' that in some ways could be described as 'caring' in a more general sense, but it wouldn't be apt.
Well, I would rather call it organization rather than just care. While the highest level of animal organization is called eusociality, humans are not hardwired to be eusocial but can display very advanced division of labor.
Now, I would not really say advantage or disadvantage. Each species have a specific role in the ecosystem and sometimes they're related in very unexpected ways. https://www.youtube.com/watch?v=ysa5OBhXz-Q
(One could argue that intelligence and brain size have more to do with evolutionary fitness, but I would speculate that intelligence is a result of increased care, not the other way around. Give a thing more time, space, and resources to grow, and it will.)
As a species, the more we collectively care for each other--through actions, not feelings--, the greater chance we have for survival and growth.