Why did he go so mad? This is technically correct, but not quite natural. "To go mad" in English usually refers to losing sanity or abusing power (Example: Our supervisor has gone mad). It is almost always used in the perfect (have you gone mad?, he has gone mad) or gerund (he's going mad). If you are talking about emotions, the most natural expression is probably "Why was he so mad?" Why did he become so angry? This is also correct, but sounds a little formal. The fact that he actually became angry might be implied. During a quick, informal conversation you may hear "Why was he so angry?"

The first is more colloquial and American-sounding, but could be used in natural conversation anywhere. The second is completely 'neutral' .However it sounds a little stilted, possibly because of the word 'become' . I'd say that 'Why did he get so angry?' is the more usual neutral phrase.

We say: Why did he get so mad? ; and Why did he get so angry?. The second sentence is more correct but used less often. The first sentence is incorrect. Although

The answers and comments you've already received are correct -- I'll just add the following: "Go mad" is mainly British, and has a similar meaning to the the American phrase "go crazy." "Get mad" is mainly American, and has a similar meaning to the international English "get angry." You could substitute "become" for "go" or "get" in the above phrases, however, it would be unnecessarily formal, as well as ambiguous in meaning (especially if you say "become mad" -- in America to get angry, but in UK, to go insane / crazy). There are also lots of slang expressions, but that can wait until another day.

Thank you, sweetieee.:)