How was Itachi/Obito able to kill this badass?

[Your Creepy Stalker]

Oh that's simple. They planned to kill the uchiha's once setsuna had died of Natural causes. Setsuna died of a heart attack, and Obito and Itachi attacked.

 

[Bad Touch Yakushi]

Most probably wasn't at that point. But you have to realize that it was a coup on the Uchiha with the intent to end it. Coups are normally done to simply overthrow an establishment. It was a surprise onslaught, which no one expected and it was done in the dead of the night.

That must also have been a factor.

Even at night I don't see that as a possibility. And I don't think there's any proof for or against him being in the clan at that point.

 

[Bad Touch Yakushi]

what kishimoto has done to the uchiha cannot be undone but instead of looking at the negative look at the positives one of the strongest ninja currently is an uchiha thats all i got haha

I know right, Setsuna is the biggest and strongest badass in the series- so cool that Kishi did that.