kindness, compassion; unkindness, harsh treatment
—While I was away on vacation, my poor garden suffered under the tender mercies of the hot summer sun. 

In one of his psalms, David appeals to God’s goodness, saying,

Remember your compassionate and faithful deeds [tender mercies and lovingkindnesses—KJV], O Lord,

for you have always acted in this manner.

Do not hold against me the sins of my youth or my rebellious acts!

Because you are faithful to me, extend to me your favor, O Lord! (Psalm 25:6,7)

Here, and elsewhere in the Old Testament, tender mercies means “kindness, compassion.” But its appearance in Proverbs gives us today’s ironic usage of the phrase, because even the “kindnesses” of an evil person are actually cruel:

A righteous person cares for the life of his animal,

but even the most compassionate acts [the tender mercies—KJV] of the wicked are cruel. (Proverbs 12:10)