Ben was grinning with delight because he knew Nathan was a goner, and thrilled to be witness to Joshua's dismissing the SOB into Barnabas' murderous grasp.
Given all the terrible things Nathan did, including being responsible for Naomi's suicide and Millicent's madness (well, along with Barnabas), he more than deserved his fate. One almost wishes Nathan's pain had been made to last longer, but I'm sure his terror over knowing what was going to happen to him was (almost) torture enough.
Love, Robin