In an act of kindness, he donated all his savings to the famous charity, Oxfam International, and hitchhiked his way through America to Alaska.

I think if we want to express a hard feeling like revenge, for instance, we can say in an act of angriness he tore the documents apart. 

This age-related trend can be observed in all cultures.

We can find in all cultures this age-related trends.

However, not every individual is the same.

Not everyone is the same.