Surely the illegitimate government would be the rebels, they're the ones rebelling against the proper rule of law. The group that won the election is the rightf

Surely the illegitimate government would be the rebels, they're the ones rebelling against the proper rule of law. The group that won the election is the rightful government. Even if you disagree, it should be plain the term is ambiguous enough that it's probably better to just not use it. What is gained by calling either group "rebel" as opposed to "armed?" With either you'd need more information to have an understanding of the situation.