As much as I want Kara to have good and supportive people in her life, she often seems doomed. Now it looks like Lena will go bad on Supergirl, especially if she teams up with Mon-El's mother Queen Rhea. After having to take down her own boyfriend in a fight, Lena might start acting out. For a Luthor, that typically doesn't end well.
Rhea, or Mom-El as I like to call her, approaches Lena Luthor at the end of the episode with a business proposition. It's almost definitely a ruse. What corporate affairs could a Daxamite have gotten into in only a few weeks? Jack's death, which happened so she could save Kara's life, is weighing hard on Lena. It seems possible that she might end up blaming Kara for the loss — however, she initially broke up with him over family loyalty. This is a complicated little situation. I think it's fitting that the show's newest villain, instead of Lena's own adopted mother, was the one to swoop in at Lena's vulnerable moment.
I love Kara and Lena's relationship, but it seems inevitable that Lena will be tempted by the dark side at least once. It's in her nature. Don't get me wrong, I also think Kara might go bad at some point in the series. Maybe they'll turn evil and twisty together.
This still doesn't quite answer what Rhea is up to on Supergirl, either. Is she just causing chaos? I like that Teri Hatcher's character is down on land for her evil deeds now, instead of on her regal ship, but how is messing with Lena Luthor going to help her get Mon-El back? Hopefully Lena won't sink too far into the villainess quicksand, and Kara will always have her in her life. But with Cadmus and Daxam over both shoulders, things are not looking up.