God’s mercy proceeds or flows from His love and extends to the vilest of sinners.
What is God’s love?
It is that Divine attribute that moves Him to freely and selflessly give himself to others for their benefit or good.
It is much more than an attitude or an emotion or a work.
It is an attribute of God, part of his very being or nature.
But why would God love us, the unloveable?
There is no reason except that He is love and it was His sovereign will and good pleasure to do so. The Lord did not set his love on you nor choose you because you were more in number than any of the peoples, for you were the fewest of all peoples.
But. Because the Lord loved you.
Why does the Lord love us?
He loves us because, He loved us.
His love originated with his infinite goodness and nothing of our own.
His love did not begin with us, so it doesn’t end with us. It begins and ends in his attribute of love, which is an infinite and immutable aspect of who he is.