Hmm, I like puzzles!
Well, my first thought is that the subject is bathed in a bright light and looks almost blissful in his facial expression, despite having been stabbed. Perhaps he is a hero to the artist who is looking to idolize him in some way? Is the letter in his hand a clue that he was doing vital work right up until his final moments? Or a hint as to why he was murdered? And does the (painters?) name beneath the subjects suggest a relationship between the two? I am so going to pull an inspector google on this and find out more!