As most arguments go, our issues may be one of emphasis. I wish to emphasize the 'out there' objectiveness of what a definition refers, and maybe you are emphasizing the 'in there' subjectiveness in how a definition is constructed. The important issue isn't the subjective definition itself, the important issue is that the definition is well-constructed (or well-defined) and that definition is applied consistently (both in terms of its popular connotation and specific usage in one's views).
***It's all in the definition :: the definition is subjective :: therefore any definition is flawed and malleable.***
I agree that any definition is 'flawed' in that any definition ultimately breaks down (e.g., it refers to undefined terms, circularity, etc). However, in a more practical light this is overkill. Definitions as long as they are well-stated and applied consistently are very useful in communication and even absolutely necessary.
Hence, using terms like 'good' and 'evil' in regards to terrorists, etc is perfectly legitimate as long as we understand (relatively speaking) exactly what that individual means and perhaps can confirm that they are applying these terms consistently. Although, they are arbitrary definitions since we could just as well re-apply the same terms to the taste of food and this may not raise any eyebrows.
Are we in agreement still?
Warm regards, Harv