so.. he would cause even further bloodshed by doing that.
Please note that only Those who resisted were actually killed while yes there where civilians who were attacked that was not done by Jaina specifically, despite what the ingame things shows, most where put into the violet hold and later on released.
"while some Silver Covenant opportunists took the chance to rob, attack, and otherwise harass the civilians of their political rivals." -
https://wow.gamepedia.com/Purge_of_Dalaran you would understand Jaina's action if you played through the whole storyline as Alliance, Garrosh as the Horde Warchief used Dalaran to get a weapon of Mass destruction, just as he had bombed theramore as the WARCHIEF of the HORDE.