Magic was hard for humans to handle until a 200 years ago when Hell's Gate fell and demonic essence spilled on the Surface (prior to that the surface held mainly angelic and human essence which naturally blocked most people's magical abilities.) There still were people who used it even back then, though. It's just quite the common thing nowadays for humans to have increased magical abilities. 

Oh yeah, an important point. The line between good and evil doesn't depend on the race. There's plenty of monstrosities the Angels have committed and plenty of well-established demon syndicates and whatnot. The major difference is that the angels don't touch the humans but view them as weak sheep while demons find value in them... and eat them. In any case, the whole thing is very grey and in the past 200 years the races have come to respect each other more and to discriminate less. Though most angels and demons probably still hate each others' guts deep down inside. :D It's just that now they have the humans to beat a healthy dose of respect into them when they go astray.