We're talking about World of Warcraft right?

I think Alliance is good and Horde is evil. Or something like that

Bioterrorism Security Assessment Alliance.

in many books it says so, but that is most illogical, he probably sold it to a impostor who seemed to be in a alliance with his evil mind

The Allied Powers were a military alliance, so the term is quite straightforward. However, you might still ask why the opposing military alliance between Germany, Austria, and Italy, was not referred to as allied powers. We tend to think of alliance as a good thing, so when we are in an alliance we are the allies; when our enemies are in an alliance, it is more of an axis of evil kind of thing. In WW II people talked about the Berlin-Rome-Tokyo Axis. It should be noted that Japan was not formally allied to Nazi Germany, although it became a kind of alliance in that both Japan and Nazi Germany were at war with the US.