It was just probably to trap and or imprison him. Wouldn't look good to kill a house of judgment member. If skolas didn't kill variks for betraying him then I doubt eramis would. Plus variks freed her from the Prison of Elders. Variks is probably more of an annoyance to her imo. Plus when variks tells her to stop, she stops, im surprised people didn't notice that. I would also imagine eramis could just crush variks or put a shrapnel launcher through the heart. But eramis is one of the smaller fallen bosses. But that's hard to tell. In the cut scene she looks about to be size of riksis the devil archon. But in the gameplay she seems to be around the size of Askor the Archon Priest.