Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Sure, The Jew of Malta is cynical and depressing. But, come on, it's also hilarious. Barabas is going around killing people, but he does it with a smile on his face and a song in his heart. He gets a lot of joy out of the planning and execution of his devious and creative plots. If he can get into the groove of things, the audience will, too.
So, along with the unfairness and hypocrisy you see at every turn, there's also a lot of morbid humor. And when we say morbid, we mean it. Take, for instance, Ithamore's remark when Barabas is mixing up the porridge with which he's going to poison Abigail and the rest of her nunnery: "Why, master, will you poison her with a mess of rice porridge that will preserve life, make her round and plump, and batten more than you are aware?" (3.4.64-66).
Well, Ithamore is playing on the idea that the nuns and friars were all getting busy making babies with each other. Ithamore's implication? Abigail is eating for two. What makes this even more macabre is that, even though Abigail is chaste, it's not because the friars have squeaky clean morals. When Abigail announces that she dies as a Christian, Friar Jacomo remarks, "Ay, and a virgin too; that grieves me most" (3.6.41).
It's ugly, it's dark, but if you can get onboard the Marlowe Ship of Macabre Jokes, you might just find yourself laughing at what T.S. calls the "savage comic humor" of this play.