Question: I noticed that "dame", which means "not good" or "don't do that", is sometimes written in manga as katakana. I was wondering, is it because katakana is used to express a strong feeling?

Sometimes, there are also other words written in katakana. In general, what are possible reasons why people would choose *katakana* for a word instead of *hiragana* or *kanji*, especially when that word is not usually spelled with *katakana*?

Answer: 
* Style - Simply as personal style
There is also the case where some words are written in katakana on signals so that everyone, even children, can notice easily and understand.
For example, the warning signals about Bear in the forest say クマ instead of 熊, or カラス (crow) instead of 烏.