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
I think it's sometimes used when it's more like a series of syllables being pronunced, rather than real Japanese. An equivalent in English would be Ginger Megg's mother saying something like "Giiingerrr!" when he's in big trouble. The English language Wikipedia says

> 
> Katakana are also sometimes used to indicate words being spoken in a
>  foreign or otherwise unusual accent, by foreign characters, robots,
>  etc. For example, in a manga, the speech of a foreign character or a
>  robot may be represented by コンニチワ konnichiwa ("hello") instead of the
>  more typical hiragana こんにちは
> 
> 
> 

I haven't heard of any examples, apart from reading that McDonald's "Mr James" originally spoke only in katakana.