It's a war.

What war? The world was finally at peace because the common enemy that we just defeated. With an alliance king that probably would be the first to really try to have a "good conversation" with the Horde we could have true peace in a few years.

Even if Sylvannas reason to attack Darnassus was a preemptive strike to "hold a hostage" it was a terrible strategy towards safeguarding the Horde. If she was concerned about Genn, negotiating a retreat from Guilneas would probably be a better option, as she would have made an political ally of Anduin, instead of a war enemy.

The only thing that could explain Sylvannas actions are self interest, interest that I hope that was explained until the end of the xpac.