It seems to have become ubiquitous: authors say 'mad' when their character is angry. This may seem like a minor nit-pick, but it's really not. Think about it.
"It's a mad, mad world!"
"It's a madhouse around here!"
"He's mad as a hatter!"
"He was like a madman, gone all postal!"
Sure, there is a secondary meaning to 'mad,' as in "Mad as hell!" but that is a special case usage, IMHO. I tend to think it implies both 'angry' and 'out of control.'
In most cases, instances of anger are best described as 'angry,' and instances of mental derangement are reserved for 'mad.'
But then again, if we can reassign various meanings to fewer and fewer words, we have less vocabulary to remember. That's a good thing, right?