Think of it this way. The US has a requirement that you be born in America in order to become president. Maul wasn't even claiming to be a Mandalorian. It would be sort of like if the British prime minister declared himself president of the US after assassinating the president, and then the VP waged a guerrilla war against the new regime. The Brit was not born in America, and does not even claim to be an American. While still technically an ally (well, until the whole assassination part), not an American. The Death Watch and the True Mandalorians had significant disagreements over what "traditionalist" meant. It does not surprise me that Death Watch would have its own differences over whether or not an outsider could become the leader of Mandalore.
As for "committing treason" she doesn't see it as committing treason as she sees Maul as an illegitimate leader. She probably sees herself as resisting a conqueror, not fighting a rightful authority.
Plus, if I remember correctly, she expresses opposition and Maul immediately orders his Mandos to kill her and her allies, so it wasn't entirely her fault that she took up arms against him (though she probably would have anyway).
As for the whole terrorist thing, I agree. She and Death Watch were the bad guys, and if she were to come up in my Mandalorian campaign, she'd be shown as such (Death Watch is definitely persona non grata with the party of 6 True Mandalorians). However, she changes over time, especially by the time we reach Rebels. We never really get inside her head all that much to find out what she thinks of Death Watch and her past actions as a part of Death Watch, but she seems to have taken a different stance by the time of her appearance in Rebels. My headcanon (prone to change based on the upcoming arc), is that she ended up making an alliance with the remaining True Mandalorians against Maul and his faction of Death Watch. It was an alliance of convenience that never ended as they ended up fighting the Empire soon afterwards. Sadly, I doubt the True Mandalorians will end up making an appearance, but that's what headcanon is for.