"Mad" varies by region. The Oxford Learner's Dictionaries website is an excellent resource for English learners. https://www.oxfordlearnersdictionaries.com/definition/english/mad_1?q=mad (especially British English) having a mind that does not work normally; mentally ill They realized that he had gone mad. (informal, especially North American English) very angry He got mad and walked out.

"He will be mad" and "He will be angry" are identical, but the first is informal and idiomatic. Someone who doesn't favour idioms or slangs will be unlikely to use 'mad' often in this sense, but in 'every-day casual street conversation' it is very common.

"He will be mad" is more serious than "he will be angry" . "Angry" is same same with "生气", but "mad" is same same with “疯".

Hi, Informally, 'He will be mad' states intense anger than 'He will be angry'. So, there's a slight difference. If you look at the literal meaning of the word 'mad', it's either a mental disorder or a situation when someone is unable to think in a clear or sensible way. Refer to the following link for further information regarding the word 'mad': https://www.merriam-webster.com/dictionary/mad

Mad and Angry are very similar to each other, however, Mad is more used in the UK than it is in the US. Angry is a more generalised form where it’s related to to one’s humour and Mad is related to a state of mind. Therefore, Mad is a synonyms for Crazy and Angry has another connotation itself. Some will say “the dog went mad”, meaning “sickly crazy”.