[Steelfish]

Is it a sin to show kindness to sinners?

Yes, if the “kindness” you speak of is to encourage the sin.

John 2:15

“So He made a whip out of cords, and drove all from the temple area, both sheep and cattle; He scattered the coins of the money changers and overturned their tables.”

You wouldn’t call this an act of “kindness”, would you?

[kevao]

Yes, if the “kindness” you speak of is to encourage the sin.

Precisely. God judges the attitude of your heart, your motives.

So if you rent to a gay couple to be kind, God won't condemn you.

If you rent to a gay couple merely to facilitate their having gay sex, then that is something else entirely.