He has a mental disorder but he is not evil.
Akihiko Kayaba has a complicated personality. He doesn’t possess the usual villainous attributes such as the desire to amass riches or to control others. He is not evil either, he just lacks empathy. Kayaba was fascinated with computers and virtual worlds from a young age. He designed Sword Art Online to be a playground first and foremost for himself, and, to make sure that players will take it seriously, he developed the real-life death mechanic. He was in SAO from the beginning, actively involved in floor clearing, even though he knew that as soon as players reach 100th floor, that will be the end of him. He didn’t care much for his life, he just wanted total fantasy VR immersion with himself as the last boss. He acknowledges Kirito’s willpower and admits defeat in a gracious and humble manner.
Holy Sword is as powerful as the other nine unique skills combined.
Holy Sword, Heathcliff’s sword skill, is used in combination with his one-handed sword and shield. Heathcliff can rapidly switch fighting styles creating impenetrable defense coupled with offensive moves. The other nine Unique Skills were meant to be acquired by players and used in the final battle on floor 100. This shows that Heathcliff actually gave players a chance to defeat him. He didn’t want to defeat the nine best SAO players, he wanted a challenging fight so he designed nine skills to stand up to his own. Only one skill was discovered, Kirito’s Dual Blades. The others were most likely erased when Sword Art Online transitioned to ALfheim Online.
Heathcliff did actually defeat Kirito.
The two fought twice: one time in the Collinia Arena and the second time on Floor 75 after The Skull Reaper boss when Kirito realized that Heathcliff is Kayaba. The first time, Heathcliff used a little cheat to win the duel. He noticed that Kirito is a strong player so he cheated with the intention of making him join the guild and observe his development. The second time they fought the final SAO battle. Heathcliff’s Holy Sword proved to be the superior skill and Kirito stood no chance. As he was dying, he gathered his strength and willpower, grabbed Asuna’s sword laying nearby, and stabbed Heathcliff.
Heathcliff designed the Medicuboid.
The device tested with Yuuki’s help, the Medicuboid, is another one of Kayaba Akihiko’s inventions. In the last episodes of SAO 2, Yuuki’s doctor mentions that the designs for the medical immersion device were given to them by Koujiro Rinko, Kayaba’s assistant. Kirito immediately reacts when he hears her name realizing that the Medicuboid is Kayaba’s brainchild.
Heathcliff helped Kirito defeat Oberon.
It seems that Kirito doesn’t fare well against villains who transform themselves into final bosses. Unlike Heathcliff that gave Kirito a fair fight, Oberon just used his game master privileges to overpower Kirito. Just as Kirito was ready to give up, Heathcliff’s ALO avatar appeared and reminded him that human will is a stronger force. But Kirito received more than a pep talk, he also got the login details for the game’s console. He quickly reduced Oberon to level one and defeated him in a fair duel. It’s obvious that Kirito couldn’t have defeated Oberon without Heathcliff’s help. This unveils another layer of Kayaba’s complex personality. Did he help Kirito because he took a liking to him or did he want to see Oberon defeated as they’ve had their differences in the past? Or maybe a bit of both?