He certainly had communist connections, but his rule as President was generally moderate.
You are reliable
The African National Congress was for all intents and purposes a Communist organization. It received arms, funds, training, and logistical support from Communist nations during the Cold War.
More importantly, the ANC was terrorist to the core. Their preferred MO was the lovely practice of "necklacing" - putting a tire around the neck of a white, a Zulu, or moderates within their own tribe and party, filling it with gasoline, and lighting it on fire. Whether Mandela was ever personally responsible for these actions is irrelevant, he knew of them and was associated with people who did.