Lena...

Yeah, Lena sucks now. And it's not really in character. Where was all this petty vindictiveness when she found out Lex had cancer? Ya know someone who actually betrayed her? The guy who would manipulate her at every opportunity, and apparently still does posthumously.
Apparently several murder attempts and kidnappings pales in comparison to the betrayal of not being told a secret. (And yeah I get that he's family, but given that her family is depicted at being at best indifferent to her, and consider love/affection a weakness. It's not clear why she would feel particularly loyal to them. They treated her far worse than Kara ever did)

I really hope someone points out that her plan is basically the same one the rogue Kryptonians had in season 1.

Oh well maybe spending time with Malefic will serve as warning. Heck maybe they share why they both want revenge and she will be forced to acknowledge that Kara hasn't done much worth avenging.
Mal: my brother erased my memory from our society because I had a disability, what did she do?
Lena: she was my friend for years and didn't tell me she was Supergirl.
Mal: then what?
Lena: that's it

We knew from the moment Lena arrived that she would eventually take a villainous turn. It was only a matter of when. The biggest question is will she be redeemed, or will she commit to her darker side.

Admittedly, it's very irksome and vexing when the writers under the EPs directives decide to make a character that is (reasonably) well-liked and turn them evil for the sake of moving a story along. It is but one of many, many poor executive decisions and technical issues that plague the show but I won't get into those things here.

Yet, as with anything on Supergirl, there is always hope. The show seems to enjoy pushing the fanbase's buttons and seeing how far they can push tolerances and then somehow managing to save themselves at the last minute. They've been doing this since the very beginning. The EPs and writers seem to thrive on creating fan division and needless adversity for their own sense of gratification. Sometimes it backfires and translates into ratings drops. Who'd have thunk it?!

I have no doubt that Lena will be somehow "saved" (since it is the theme of this season's arc) from being a full-on Luthor but it may off-put a good chunk of the viewing base in the process. Like you, I now tend to watch the episodes more to see which rabbit hole the show will take us. Some have been surprisingly enjoyable (primarily in past seasons), but many have been very trying and exasperating to go through. In all fairness, the actors and various crews are doing their very best to give us a quality viewing product on a weekly basis, but they are being hampered by their own wonky writing staff and questionable executive vision and fuzzy direction.

If you're truly upset about it, I'd suggest paying a visit to my thread "Rom's Rage Cage" to vent your frustrations there. That's why I opened it.