This means like a fire burns in an oven, these people have strong evil desires within them. The people's desires are represented by their "hearts." (See: [[rc://en/ta/man/translate/figs-simile]] and [[rc://en/ta/man/translate/figs-metonymy]])
The word "smolders" means something is burning slowly without a flame. Alternate translation: "Their anger grows slowly and quietly" (See: [[rc://en/ta/man/translate/figs-metaphor]])
The intensity of their anger is spoken of as if it was a very hot fire. Alternate translation: "it gets very intense" (See: [[rc://en/ta/man/translate/figs-simile]])