Any emotion left uncontrolled becomes a problem. Not only anger. The reason anger is singled out is due to the predominance of passive mindset.
There are two ways in which a passive mindset can judge anger as being fundamentally bad:
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Because people who are angry are quite likely to cause harm to others, and because passive people judge the worth of actions based on the kind of effect they would have if they were directed at them, anger is judged as negative. In plain terms, anger is bad because someone who is angry at you makes you feel uncomfortable.
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Because anger is mentally and physically demanding, and because passive people are mentally and physically weak, they have no experience of healthy anger. Instead, they only have experience of frustrated and/or lightheaded anger. Furthermore, if they have no memory of healthy anger, and if they cannot resolve frustrated and/or lightheaded anger without eliminating anger altogether, then they have no choice but to confuse the two: frustration (and/or lightheadedness) and anger. Thus, they think, anger is bad, because anger is equal to frustration (and/or lightheadedness), since when you eliminate anger you eliminate frustration/lightheadedness (but you also eliminate frustration/lightheadedness when you eliminate life, and yet, few argue that life equals frustration/lightheadedness.)
Anger, like every other emotion, is neutral per se. It acquires value only when you relate it to its actor, namely, when you measure the effect it has on its actor.
Anger, like every other emotion, is bad only when it is uncontrolled or misdirected.