So as we all know, Scar got ticked off because Simba was born and was next in line for the throne. If I'm not mistaken, here is how royalty works:
The Prince/Princess is descendant of the King and Queen, and is next in line for the throne.
Since when did the King's brother ever come into the picture? In my mind, what Scar apparently expected was something that wouldn't be very likely to happen (unless of course, he caused it). So don't you think it was quite a logical fail (or just greediness) on his part to expect to be King when his brother was already King and had a mate? (btw i think it was also inevitable that Mufasa would find a mate)?