To be honest, her goal wasn't to kill(though it would have been a happy side effect). It was to take him down a peg or two and make him realize that maybe he should start thinking.

Removing someone as a threat doesn't mean to only kill. If he had survived the attack, but calmed down some, she'd have been fine with it.

As for attacking his Drones, the last she saw of him through the Servitor, was over thirty Drones, all heavily armed and armored, and that he was still alive, and very likely going to try and kill her.

What was she supposed to do? Sit back, relax, and wait for them to point guns at her vulnerable self? He has an army. She only had two options. To either leave AMEN right then(Which I hadn't wanted to do since she's my only AMEN PC, but now realize it would have been best because of all of this), or try to at least level the playing field. Again, no one sees Riv as a reasonable person IC. It's not like she could have just walked up to him and said "Dial it down before someone shoots you" and expect any other result other than a snide threatening remark and him saying let them try to get through his army.

As it is, she's still severely out-powered here, and is constantly threatened.