ISIS is not a legitimate government. They are a group of people who are spreading chaos, and killing mercilessly in the name of the religion. They are trying to overthrow existing, established regimes (those of Iraq and Syria). It is prohibited Islamically what they are doing.

The Saudi government is a legitimate government, regardless of their current shortcomings, or things that they can do better. They govern a stable country, and are keeping peace and stability within the country. It is prohibited to overthrow them.

Here is just one example of how ISIS is being denounced in the Middle East (unfortunately, there are spelling and grammar mistakes in the captions): https://www.youtube.com/watch?v=JQ2HlIYRvCM