I saw Matadi as a youngster at Twycross where he was quite exuberant, but I also saw him after he had been injured by Oumbi (who was introduced as the silverback after Sekondi's death). I think Oumbi must have been nervous in the new situation and maybe Matadi irritated him.
His move to Howletts was unfortunate, because he was housed next to Dzanghou's group and Dzanghou reacted by deliberately intimidating him - perfectly natural behaviour behaviour when a smaller unfamiliar male appears next to his territory. Obviously even the gorilla experts at Howletts got things wrong, and they modified both enclosures to visually separate Dzanghou from Sammi (Matadi's replacement). In retrospect I am glad I didn't visit Howletts while Matadi was there. The unfortunate incident with Kiki has been explained already by
@ShonenJake13, in retrospect it may well be another husbandry failure. I am glad to say that I did see Matadi which his group at Antwerp last year: he seems very settled and I think that many Antwerp visitors are delighted by the way he sometimes plays with his little daughter Thandie.