Argument that God Of Old Testament isn't the Same as New Testament

There's an agnostic that I know who makes the argument that the God of the Old Testament was a very proactive sort of God in the Old Testament, smiting and whatnot. But nowadays, all these "horrible" things that were punished back then, are being allowed to run rampant over all the Earth. He does not buy the idea that it is now God's will that these things are happening or that God works in mysterious ways.

Is there a better approach on going about and explaining this?

Thanks for any and all advice!

God bless!