In "Wonderful World", Soo Hyun is a famous writer that suddenly loses her son in a car accident. The killer didn't show any remorse and didn't even pay for his crime, Soo Hyun lost her mind and in a moment of rage, kill her son's killer and goes to prison.
When she gets out, Soo Hyun finds out that the person she killed wasn't the real culprit and the truth involves a conspiracy. There's also Seon Yul, who's connected to all of that.

I don't think what Soo Hyun did was wrong. Her son was dead and the culprit wasn't even sorry, all she wanted was an apology and in return, she was humiliated. I think I would do the same thing, it's a human reaction, that man needed to die.
We learn in the following episodes that the man is Seon Yul's father and that he approached Soo Hyun to get his revenge, but let's be honest, there was nothing to but avenged, his father was the wrong one. Even that he was following orders, Seon Yul's father was the person that eventually killed Soo Hyun's son, he was still alive when the car hit him. I understand Seon Yul's motives, when he lost his father, he basically lost everything and he needed someone to blame and that was Soo Hyun.

The drama's message is really beautiful, that we need to learn to forgive and move on, instead of holding on to the pain, because that way, we only hurt ourselves.
The acting is flawless, specially Cha Eun Woo's, he is glowing in a totally different role, something he never did before, since he was focused on romantic comedies.