In the Venice painting, why is it that Magenta could defeat the Grim Reaper with just a shove? Because that's not the real Grim Reaper. Just as the people in the painting are an imitation of life, this Grim Reaper is only an imitation of death.
Why was the Grim Reaper after Quill? Simple, because he wasn't after Quill, he was after his satchel. And what was in his satchel? The crushed remains of his poor friend Gum. The Grim Reaper was just trying to claim Gum's soul.
It's been said that "Artists use lies to tell the truth, while politicians use lies to cover it up." On one hand, Ramo tells Claire her face is colored, even though her face is unpainted. This is because he's using lies to tell the truth about how beautiful she is to him. On the other hand, we have the Great Candlestick who keeps feeding the Alldunns lies about how it's okay to persecute the Halfies and Sketchies. As their leader (a politician), he's using lies to cover up the truth about how the other two factions are people in their own rights.
As demonstrated with the other paintings he destroyed when (supposedly) he had a really bad day, it raises an unsettling question: what would happen if the Painter had another such bad day and makes the rash decision of destroying his other paintings, including the one Claire and Ramo live in?
