I just watched the latest episode of Code Geass R2. I have come to a simple conclusion about this character during this episode.
Arthur is the single most maliciously evil creature in the entire series. Arthur has seemingly dedicated all 9 of his lives for the singular purpose of making Suzaku's life as miserable as possible.
Its not enough that he tries to give Suzaku physical pain at every possible occasion. No, no, it is his full intention to try and traumatize and kill Suzaku.
Arthur enjoys a privileged position of being untouchable by the target of his torture due to taking advantage the bond between Suzaku and Euphie. With this advantage, he does everything in his power to try and kill Suzaku. In season 1 he tried to make Suzaku fall to his death. In season 2, he tries to get him run over by a KnightMare.
Furthermore, he goes for giving Suzaku emotional trauma as well. We saw that with him stealing another precious article of Euphie's. This just goes to further twist the dagger, all part of his plans to leave Suzaku a hollowed out shell of a man before delivering the death blow.
But Arthur treats his enemy with the greatest respect. He will be the one to kill Suzaku. No one else. That is why he seemingly spared his life last season. It’s not enough for Suzaku to die; Arthur has to personally kill him. That is his desire and his lust.
Furthering my theory. The reason he took Zero's helmet last season, besides of course the whole thing of trying to make Suzaku fall to his death as Scar did to Mufasa, he did it to personally tell Zero, "I am more evil than you, you are going to have to step up your game if you want to earn this mask."