THe idiom "they were furious in their hearts" is an idiom for making a person extremely angry. Alternate translation: "were extremely angry" or "became very angry" (See: [[rc://en/ta/man/jit/figs-idiom]])
This action expressed their strong anger at Stephen or hatred of Stephen. Alternate translation: "they became so angry that they ground their teeth together" or "moved their teeth back and forth as they looked at Stephen" (See: [[rc://en/ta/man/jit/translate-symaction]])