The incident happened outside a Busan court.
The suspect surnamed Hong is accused of murdering the victim surnamed Jo, also in his 50s, in front of the Busan District Court in Yeonje-gu, Busan, at around 9:52 a.m. on Thursday, reported The Korea Herald.
The accused escaped from the scene after the incident.
He was later arrested from Gyeongju.
The two had been at odds for several years, openly criticizing each other through videos on their respective YouTube channels, both of which have subscribers in the thousands, The Korea Herald reported.
An investigation found that Hong purchased a 35-centimeter-long kitchen knife a day before the court appearance.