Anti-Korean reprisals would be likely, but why would there any plans for full-scale genocide of Koreans? Korea was considered an important and integral part of the Japanese empire- depopulating the country doesn't seem like it'd be conducive to their plans of dominating East Asia.
Attempted genocide is the extreme end of the continuum of possible Japanese reactions.
I would suspect that Koreans residing in Japan rather than Korea would bear the brunt of reprisals, since that's where Lee Bong Chang did the deed and where there would be lynch mobs against a vulnerable and outnumbered population. In Korea itself, extreme reprisaling invites broad-based revolt, which can be bad for economic exploitation.
On the lower end of the continuum of reprisals would be mob violence against Koreans in Japan and reversal of any of the reforms of the more lenient 1920s [I figure they were probably all reversed by the time we got to the Pacific War, but I don't know when, if ever, in the 1930s, things went back to the pre-1919 military police rule - regardless rollback would be accelerated].
Anger at the Koreans would be high enough that championing ethnic Koreans in Manchuria would not be a policy (as it was in OTL, albeit for instrumental reasons).
A real attempt at genocide would be a big effort provoking guerrilla resistance at a far higher level than what the Japanese experienced in OTL. It would also lead to mass Korean exodus (nowhere near a majority of peninsular inhabitants) to places beyond Japanese control. It could seriously mess with Tokyo's appetite to fight for a larger sphere in China outside Manchuria.