Other reviews cover the storyline and what you can expect from the script, production values, etc. So I'm just going to give you my overall thoughts.

The problem with setting up an addictive, shocking premise in a story is how to stick the landing. When the stakes are so high, viewers tune in to find out (how in the world) everything could possibly work out for these characters in the end. Sometimes, like Flower of Evil, everything builds beautifully until the very end. The problems are solved too easily and with too little ramifications for the main characters, which ultimately feels like a letdown because you realized the stakes were never actually that high. However, this show leading up to the end was so good, I cut the ending some slack.

Lee Joon Gi was phenomenal the whole show, start to finish.
Moon Chae Won was also great, but her performance started to feel a little too wide-eyed and naive toward the end. I needed something more from her, a bigger, more adult reaction. She didn't quite deliver.

The chemistry between the ML and FL was great 90% of the time, but-- again-- toward the end it faltered, and their scenes lacked the earlier crackling electricity. It's almost like, when they got to the heavy stuff, they felt less comfortable acting together.

Overall, I really, really liked it.