I won’t claim to know inside Bart Ehrman’s head or heart, but surely he’s read Jesus’s parable of the wheat and tares:
“As the weeds are pulled up and burned in the fire, so it will be at the end of the age. The Son of Man will send out his angels, and they will weed out of his kingdom everything that causes sin and all who do evil. They will throw them into the blazing furnace, where there will be weeping and gnashing of teeth. Then the righteous will shine like the sun in the kingdom of their Father. Whoever has ears, let them hear.”
Matthew 13:40-43 NIV
In my reading and understanding of this passage, it seems to imply that Jesus teaches that hell is not pleasant. There’s also the story of the rich man and Lazarus:
“The time came when the beggar died and the angels carried him to Abraham’s side. The rich man also died and was buried. In Hades, where he was in torment, he looked up and saw Abraham far away, with Lazarus by his side. So he called to him, ‘Father Abraham, have pity on me and send Lazarus to dip the tip of his finger in water and cool my tongue, because I am in agony in this fire.’”
Luke 16:22-24 NIV
I’m not a scholar like Ehrman is, but it’s pretty clear that hell isn’t simply darkness or nothingness.