And it's not just religious Sunnis that hate him (because he runs a secular state for example), in fact secular Sunnis, non-religious Sunnis, and pretty much every single group in Arab society including a great deal of Arab nationalists hate the guy.
It just shows you how far propaganda can do. The same people clapped for Saddam Hussein because he was, as they say, "a lion and a hero of the Arabs against the outside forces". To me it all comes down to:
1) Extreme propaganda and billion dollar Qatari/Saudi money fueling this hatred in terms of news media coverage (fake news, propaganda when reporting the Assad crimes in the form if fake photos, videos etc...).
2) Assad is not a Sunni. Saudi Arabia has been big on promoting anti-Shi'a propaganda (particularly after the Hezbollah victories) because they fear increasing power and sympathy towards Shi'i movements or Iran. So by producing this generation which has a huge anti-Shi'a mindset not just in the religious realm (this is centuries old) but in the modern day political world, everything that is remotely Shi'i will be demonised (unless the Shi'i is a sellout) and you will see tons of conspiracy theories against the Shi'a the same way some unbelievers and anti-Islam propagandists talk about Islam. It's sad and pathetic.
3) It is within the interests of the imperialist powers to have Muslims fight each other (sectarian conflicts) and to encourage such revolutions especially against governments they view as adversaries or potential adversaries, especially in a sensitive part of the world such as Syria.
4) To destroy the Syria-Iran alliance which is currently the only alliance in the Middle East which commits itself to the Palestinian cause.