There is no scenario in which Ashley shoots Wrex without you specifically ordering her to do it. There is also no scenario in which Wrex doesn't eventually stand down if you talk it out with him. So you're the son of a bitch in this situation, not Ashley.

Also, it's Timbuktu. I can see that's totally lost on you.

Edit: Oh correction, Ashley will shoot Wrex without orders if you didn't complete Wrex's personal quest. So it's still your own fault.

She only shoots wrex if you don't choose a speechcheck option or your order her too. It's all on you.