(Yeah... nice tongue.)
I think I can safely say that he himself is batmonkey insane, and he doesn't hesitate to show it. Now, initially he was not like this (duh), and his 'light' sight is more cool and collected, and even nice. This other personality was formed due to extreme abuse from his father and an intense level of pressure put onto him as a child which eventually made him snap.
This other personality only came out two times in his life, once to deal with the lovely, lovely pain of having a tattoo forcibly carved into his back as a small child, and then when his 'brother' was put in danger when his father tried to kill him. And oh boy did this kid get his revenge. He skinned his father alive, and then when the normal Malik returned he had a nervous breakdown because he couldn't remember it. Yami Malik returned roughly six years later when Malik dealt with another extremely stressful situation where his 'brother' was almost killed.
This lovely little Malik sees everything as a game, almost like a child, and gleefully takes his wins through force. In some way, he is a child (a deranged, insane child) as he has only existed for seven years and he has only experienced extremely traumatic incidents, so I think it's safe to say he'd go a little coo-coo.
I feel like pointing out (just for you folks out there) that his hair probably doesn't actually spike up and veins don't actually bulge in his face when he comes out, and that's just the animators way of showing that he is the more dangerous personality and not the ordinary Malik. If we casually learned how to jump into other anime universes, he would probably look the same but his personality would be batmonkey insane. Just felt I would toss that out for you, but I can safely say it definitely adds to showing just how crazy he is.
This villain in undoubtedly the most unsettling out of the ones in the Yu-Gi-Oh series, mainly because he has no true purpose other than to cause havoc because he finds it fun. He has probably known nothing more than people hurting him, so naturally he doesn't give a monkey bear about hurting them back. And if he beats you in a game, you go straight to Hell. Lovely, no? I heard the Summers were great down there.
Anyway, Yami Malik seems to be ultimately defeated by his other half, although we all know that he was most likely only defeated for a short while and the chances are he's going to be coming back once Malik let's his guard down, and let's just say he can kill with more than a Millennium Rod...
(I kinda actually wanna give him a hug... mind you, who said I was sane?)