He owned the company and it's subsidiaries like the army. The Belgian king had little real power so he decided to get rich off the exploitation of Africa's interior.

Leopold established control over a foreign territory with the explicit purpose of advancing his own wealth, which is the definition of colonialism. The thing is, as a king, he didn't do it for the nation of Belgium, but rather for his personal gain. The Belgian government had little to no say in how he ruled what essentialy became his private territory.

As such, it's both colonialism and a single person at the root of this all.