If I do a kind thing partly to please Allah and partly because I want people to think well of me, is the whole thing spoiled?

The short answer is no, the whole thing is not spoiled. But something is lost, and it is worth knowing exactly what.

Allah has told us plainly what He wants of our worship and our kindness: that the deed be for Him alone, with no other master seated beside Him in the heart.

“So whoever hopes for the meeting with his Lord, let him do righteous deeds and associate none in the worship of his Lord.”

Qur’an, Al-Kahf 18:110 · meaning

And the Prophet, peace be upon him, passed on these words of his Lord, which show how the ledger actually works.

“I am so self-sufficient that I am in no need of a partner. Whoever does a deed for My sake and someone else’s, I leave him with the one he associated with Me.”

The Prophet (peace be upon him), a Hadith Qudsi, Sahih Muslim (2985) · meaning

Notice what is said here and what is not. Allah does not say He destroys the deed or punishes you for the mixture. He says He leaves you with the one you set beside Him. The share you gave to people becomes their affair and carries no weight; what was truly for Allah stays with Him. So a kindness done mostly for His sake, with a thread of wanting to be thought well of, is not wiped out. The thread is simply worth nothing.

Sincerity is not a peak you climb once and are done with. It is closer to a garden you tend all your life. The stray thought of what people will think comes to everyone, and the work is not to never have it, but to notice it and turn the deed back toward Allah, the way you turn a plant to face the sun. The righteous before us walked this same road. So do not throw away the good you have already done, and do not let this worry stop the good you meant to do tomorrow.

One small step: Before your next act of kindness, say in your heart just once, “O Allah, this is for You,” then do the deed and let it go, without checking who noticed.