And especially that the politicians from his party managed to (after some time, convincing and protests, but still) overcome their affiliation and personal political interests to impeach the president.
Some of that may be self-interest given that the President tried to imprison the heads of his own party during his attempted coup. The army deserves at least some praise for refusing to escalate the situation and ignoring orders to invade their own parliament.
Sadly very few politicians from his party voted for impeachment (although more than zero). Goes to show how if only his party had 15 more members he might not be getting impeached.
Dunno what's wrong with this guy, I understand he's a former prosecutor who locked up presidents Lee and Park. Now he somehow repeats the same mistakes, declares martial law, sends the army to the Parliament to arrest the uncooperating opposition? And it all backfires because the National Assembly can always override the President. He'll probably end up in prison. He should have known better. What is wrong Korean politicians (both ROK and obviously DPRK) and abuse of power? At least in the ROK they have a functional democracy with at least two former convicted presidents.
I've read that he had cronies in the military. My only guess is that he was hoping they'd back him hard enough to suppress the opposition. (Either by ignoring the vote, or by preventing them from voting in the first place--which it seems came close to happening.)
It wouldn't have worked anyway. The ROK has a history of right wing patries abusing power and violent student protests clashing with riot control police. That is hard earned democracy. If he thought the people wouldn't be out in the streets backing up the Assembly, he was so wrong. They're out in the streets protesting peacefully with k-pop light sticks as we speak.
Just for reference, in the US there were protests after protests in late 2016 and early 2017. The only difference they made is tiring out the participants.
Korea also saw almost nonstop protests since Yoon came to power - TBH I think there was always some part of Korea protesting the government, since forever. But you can't just kick out a president for being hugely unpopular - they must do something so drastically and flagrantly unconstitutional that the majority of citizens must say "Yep that's unconstitutional, we have to drag him down because our Republic is at danger."
Whether you like Trump or not (I don't), Trump of 2016/2017 did not meet the threshold. (Half of America had just elected him, what would you expect?) I think Jan 6th clearly met the threshold, and Biden should have seen that Trump ends up in prison. He somehow didn't - I get that precedents are important in politics and America had never seen an ex-president in prison, so would have been very controversial - but I think it was a wrong decision and America is now paying the price.
In 2016/2017 there was much more awareness of Russian interference than there is now. The protests were more about legitimacy than about disagreement or dislike.
By 2021 the Putin/MAGA forces had accreted so much power that it was safer for Republican senators to bend the knee and acquit in the impeachement trial.
The story is even wilder: because Yoon was a former prosecutor who went after the previous conservative president Park, when Moon (the previous liberal president) appointed Yoon as chief prosecutor, it was initially widely opposed by conservatives.
And what did he do? Almost immediately after becoming chief prosecutor, he starts hunting down Cho Kuk, his superior (minister of law) who Moon just appointed, and Cho's whole family members, in a blatantly motivated investigation. And also doing the same for Lee Jaemyung, the current opposition leader, who is now one supreme court verdict away from becoming ineligible for the next presidential election, for violating some election law.
(One can argue for days whether Cho and Lee did crimes, maybe they did some, but it's clear that if prosecutors were 10% as enthusiastic in investigating other politicians, half of the current legislative and executive branch would be out of jobs.)
...which gave him a gigantic publicity boost as "defender of law and order," but it was always built on hypocrisy and Korean prosecutors' ability to take care of their own first. We knew Yoon was a hypocrite, but nobody expected him to be crazy enough to start a coup. In one drunken decision Yoon apparently handed presidency to his archenemy Lee Jaemyung, who's also known for being rather divisive and not forgiving his enemies. Talk about karma.