情けは人のためならず (なさけはひとのためならず): the good you do for others is good you do yourself; those who are kind benefit themselves; compassion is not for other people's benefit

the good you do for others is good you do yourself; those who are kind benefit themselves; compassion is not for other people's benefit

Widely misread by native speakers too. It does not mean kindness spoils people: it means the good you do comes back to you.

A Japanese proverb (ことわざ)