Mercy is giving them a piece of your heart, not a "piece of your mind".
There's a legend about a rabbi who welcomed a weary traveler into his home for a night of rest. After learning that his guest was almost a hundred years old, the rabbi asked about his religious beliefs. The man replied, "I'm an atheist." Infuriated, the rabbi ordered the man out, saying, "I cannot keep an atheist in my house." Without a word, the elderly man hobbled out into the darkness.
The rabbi was reading the Scriptures when he heard a voice, "Son, why did you throw that old man out?"
"Because he is an atheist, and I cannot endure him overnight!"
The voice replied, "I have endured him for almost a hundred years." The rabbi rushed out, brought the old man back, and treated him with kindness.
When we treat unbelievers with contempt, we're not serving God. He wants us to love them as He has loved us.