After WWII, the southern part of Korean was under American occupation (it was previously under Japanese occupation, but Japan surrendered to the US). It was the US occupation that installed installed the precursor of current government, with elections which had a lot of boycotts/opposition/oppression. Right wing killed a few hundred thousands locals during these times.

To be clear: I'm not justifying anything here, or claiming that you should share this viewpoint. I'm answering your question: how could NK possibly see the US as an aggressor.