The killing vote starts from a very clever premise: the justice system is corrupt and inefficient, the people's justice will find and punish devils deemed innocent. Sometimes the story drags too much, some of the dialogue is forced, but the drama is there, some of the actors are really outstanding, and the others are good. There are many twists and turns, many of which are simply brilliant, while others were unnecessary. But overall, it is a series that will make you want to watch it to the end as the story progresses.
I disagree with the reviews of those who criticize the moments of levity in the series. These moments are well contextualized with the characters. You will never see the "professor" (the most dramatic character in the series) laughing, just as you will never see the antagonists making jokes. The series also lends itself to interesting philosophical debates on justice, the writers have done their homework in that Cesare Beccaria is also quoted in the series, and several references to modern digital rights.
The weak point of the series is the ending, but I can understand the decision of the writers to keep things a little open to propose a second season. I strongly suggest watching it in the original language with subtitles; the Korean language is very expressive, so watching it dubbed would kill much of the acting.