Is it state level activity or just corruption? What do you think the actual relationship between ISIS and Assad is?
Probably some of both, but Daesh (sidenote: I always use this epithet for them since they apparently don't like it) definitely has a solid history with the Assad gov
going back to their days in Iraq as AQI, and was accused by the highest-regime defector of controlling
what would become Daesh.
The regime also took efforts to stoke the fires of Islamist insurgency very early on in the war, and has continued that by not interfering with Daesh-held territory for the most part (and certainly not to the same extent they do with other groups). Kyle Orton provides a good overview of the overall support to Daesh and other Islamists provided to the regime
here, and it's worth a read given the time.
My overall understanding of the relationship between Daesh and the al-Assad regime is that they are de facto allies, with perhaps some more affiliation at the higher levels. The al-Assad regime views Daesh as a tactical ally; Daesh has been the most effective force in the field at destroying moderate and non-Daesh Islamist rebels, and for the most part, Daesh leaves the regime alone to focus on closer threats. The regime probably appreciates that they will have to deal with later, but thinks Daesh can buy them time (and also serves as a useful strawman to paint all of the opposition as fanatical Islamists). To quote the Der Spiegel article on
Daesh's structure (another fantastic read on the subject):
The regime's relationship with Islamic State is -- just as it was to its predecessor a decade prior -- marked by a completely tactical pragmatism. Both sides are trying to use the other in the assumption that it will emerge as the stronger power, able to defeat the discrete collaborator of yesterday.
I'm not entirely convinced by the argument that Assad created Daesh; I find some credence to the idea that the GSD co-opted (or thought it co-opted) a group that it already had ties to (from when it was AQI) and unleashed them as provocateurs (as the Algerians did, and as the UDBA did, and as the Cubans still do to Cuban-American groups). However, things like
Dabiq magazine don't make sense in that role; to me it argues that Daesh really does believe that it can and will establish a caliphate (realistically, it already has) and eventually overthrow the al-Assad regime. I'll admit there's a possibility they started out as provocateurs from the regime but have moved more independent-minded on the belief that the regime can never again re-take control of the country. Or perhaps they are just hedging their bets. Maybe they learned in Iraq how powerful and how easy it is to get foreigners to come join their forces and view them as a powerful source of expendable bodies, and decided to keep it up.
Who knows? In summary I certainly don't think of them as enemies in any tactical or operational sense; maybe in a long-term strategic one, but I doubt that many people in Syria are thinking too long-term right now anyway.