Let's look at the classified documents between Kim Il-sung and the Soviet Ambassador immediately after the assassination

[Exclusive Diplomatic Documents] October 1974: To the Soviet Ambassador, "Return to the DJ followers within the Min-dan to which Moon Se-kwang belongs"
News Free Zone
Circumstances have been confirmed through diplomatic documents from East Germany that North Korean leader Kim Il-sung attempted to shift the responsibility for the August 15, 1974, assassination of the late First Lady Yuk Young-soo onto the camp of former President Kim Dae-jung (DJ). Yonhap News reported on the 4th that Seong Sang-hwan, a professor of German Education at Seoul National University (Director of the Kyujanggak Institute for Korean Studies at SNU), discovered these in the German Foreign Office's political archives.
There are records of a meeting held on October 25, 1974—immediately following the August 15, 1974, assassination of First Lady Yuk Young-soo—between North Korean leader Kim Il-sung and the newly appointed Soviet Ambassador to North Korea, Gleb Krulikov. [1, 2]
These transcripts were declassified in the mid-2000s but remained unknown to the public until a Seoul National University research team unearthed them from the German Foreign Office's political archives in 2020, revealing the details.
There was once a massive conspiracy theory that it wasn't North Korea invading the South, but South Korea invading the North, but that completely vanished once the Soviet classified documents were released.
The article above is a document regarding Kim Il-sung's meeting or report to the Soviet Union immediately after the incident; give it a read.