Body plan
First off, let's talk body plan. Your demons, being roughly human-sized, will need very large wings. Starting with the similarly sized pteranodon, they'd need around a 20 foot wingspan to support a 100lb body. (Very roughly: pterosaur weight estimates are generally somewhat dubious.) Your demons have to support the added weight of horns and arms, so you might need something a bit bigger than that.
The body, in general, will be lightly built, with short, but strong, legs, and probably relatively long arms. They'll almost certainly have massive pectoral muscles, relative to humans, to support their wings, with a second, smaller, set of pectoral muscles to support their arms.
Bat-like wings are certainly possible. The primary difference between bat and pterosaur wings is that bats rely heavily on an enlarged second finger (our equivalent is the pointer finger), while pterosaurs relied on an enlarged fourth finger. In any case, convergent evolution will likely drive your demons to have a substantially more enlarged primary flight finger, as compared to bats, with relatively thin secondary flight fingers to save weight.
Behavior
Your demons will likely have a diet heavily reliant on carrion and medium sized animals or fish. (Or, at least, that's what their ancestors would've eaten.) This, again, is based largely off of pteosaurs, which are the most similar creature to what you're designing. Pterosaurs used long skulls to pluck food from the ground mid-flight. Your demons have short, human-like skulls, but are hexapod, unlike pterosaurs, with a secondary set of arms beyond the ones that have evolved into wings. These arms will likely serve a similar purpose, growing long, with grasping claws, to pluck prey either from the earth or from the sea. Flying also makes finding carrion easy, so they'll probably eat a lot of that.
Takeoff will be from a quadrupedal stance. This is the same way that pterosaurs likely took off, and is a bit more efficient than the two legged launches that birds favor. They'll be using their arms and legs to launch, rather than legs and wings, like pterosaurs favored. They likely won't be able to walk upright, but will instead walk on feet and knuckles, similar to gorillas, likely squatting on their back legs, supported by their tails, when they aren't moving around. This will provide them with the capacity for short bursts of speed for takeoff, while protecting their hunting claws. If they're living in a rocky, volcanic environment, of course, they'll likely be using their hands and claws to climb when on the ground, and rely on launching off of cliffs for takeoff.
There isn't much, in terms of food, in high temperature areas with lots of volcanic fumes. However, if the only nesting areas are active volcanoes, it's reasonable that your pterosaur-demons would evolve increased resistance to heat and toxic gasses in order to live there. It's also possible that volcanic nests are simply the best way to avoid predators. If their surroundings are largely treeless grasslands with both plentiful food and plentiful terrestrial predators, raising young in a largely inaccessible area may be advantageous enough for them to evolve to dwell in those areas.
Horns
Horns, in general, aren't terribly useful for either flying creatures or predators. However, they aren't completely unheard of.
It's likely that either the horns will be relatively small, such as those found on carnotaurus, or else that they'll be hollow, such as the casques found on some species of hornbill. In the former case, they'd likely be used in dominance fights between males. In the latter case, they'll likely be used as resonating chambers for calling out across great distances. The latter seems much more likely, given that they've got arms, rendering headbutting somewhat less useful as a means of combat. The ability to produce loud sounds, for a creature living largely in a low-visibility, loud environment such as the inside of an active volcano, would certainly be useful. Demons could call to one another, both for general communication, and to gauge distances for landing. Swooping in to a perch through a dense cloud of volcanic smoke would certainly be much easier for a creature that can call back and forth to an already-perched mate or flockmate, in order to effectively gauge distances, and figure out which direction "home" is in.
