In the spirit of the Midsummer Fire Festival, We're going to take a look at the Elemental Lords, one of which was the inspiration for this holiday. The Elemental Lords were originally agents of the Old Gods, and some of their most powerful ones at that. They were used to do little more than sew chaos in Azeroth and keep it in a constant state of disorder.
As we know, the Old Gods didn't hold up very well against the Titans and their champion, Sargeras. When the Old Gods were beaten, bloodied, and chained far beneath the earth, their Elemental Lieutenants received similar treatment. All four of them were banished from the plane of mortals, trapped within an Elemenal Plane, where they could cause as much trouble amongst themselves as they wanted without bothering Azeroth.
Of course, long after the Pantheon no longer had an active hand on Azeroth, the mortal races did catastrophically dumb things as they usually do and ruined it for everybody.
There are four Elemental Lords, each ruling over their own domain. These lords are Ragnaros the Firelord, Neptulon the Tidehunter, Al'Akir the Windlord, and Therazane the Stonemother. I think you can figure out their elements fairly easily. If not... well, I'll be nice and lend a hand. Ragnaros is Fire, Neptulon is Water, Al'Akir is Air, and Therazane is Earth. Slight variations on those elements are still, at their base, that element. Frost will still fall in Neptulon's domain, and things like storms are covered by Al'Akir. Does Neptulon still hold domain over the rain during one of Al'Akir's storms? Probably, but debating elements for hours will just get confusing, so let's smile, nod, and move on.
For the most part, the Elemental Lords are not actively malicious now that they are seperate from the Old Gods. They are, in the end, little more than the embodiment of their element given sentience. In nature, the elements work together to craft beautiful landscapes and things like that, but when you really get down to it, those landscapes are crafted by the elements beating against one another. Wind and water erode the earth to form rivers, fire burns away brush and vegetation, which later makes the land more fertile, et cetera. This is basically how the actions of the Elemental Lords play out, except they have sentience behind them.
It is easy to see the Elemental Lords as evil, because their actions put massive amounts of people in danger, but that's the same in our world except without that sentient monstrosity thing. Hurricanes, tornadoes, mudslides, earthquakes, wildfires, are all naturally occurring disasters that harm many people. Nature isn't evil. It simply is what it is.
Let's take a look at what we know about each of the Elemental Lords. How much we know varies wildly between them, but we'll do what we can.
Therazane the Stonemother
Therazane is... not a very pretty individual, but she is the embodiment of Mother Nature. If you really want to know what she looks like, she is very close in appearance to her daughter, who you can see in Maraudon. Therazane herself is a very caring, motherly type. It is hard to make her angry, though she constantly feels threatened by the other Lords. After all, it is her domain that takes the most damage from the other elements gaining power. Due to her kinder, gentler nature, she doesn't seem to have as much of an active hand in current day Azeroth as some of the other Lords.
It is a popular belief that Therazane is the true identity of the Earth Mother the Tauren believe so strongly in. She fits the description quite well, the only issue being there is nothing beyond that tying the two together. Until we get a few clearer hints, all we can do is guess.
Al'Akir the Windlord
Al'Akir is regarded as one of the weakest Elemental Lords, but not necessarily because of a lack of strength. He embodies the wind fairly well. He comes and he goes. Some days he's especially active and ferocious, other days he just doesn't give a damn.
It is probably due to this that Al'Akir's son is better known than he is, and only thanks to the Firelord. After the fall of the Old Gods, Ragnaros was the first to make his move against the other elements. For one reason or another, he targeted Thunderaan, the son of the Windlord. His goal was not simply to destroy the Windlord, but to consume him. Ragnaros' most trusted lieutenants, Garr and Geddon, ambushed the Prince of Air. Ragnaros consumed most of him, but his base essence could not be destroyed. Thunderaan was instead bound to an amulet, which was then sundered in two. Garr and Geddon held onto those pieces with great care. And then adventurers killed them, reforged the amulet, freed Thunderaan, then killed him for his sword.
To be fair, Thunderfury is sweet.
Ragnaros the Firelord
Ragnaros is one of the most militant of the Elemental Lords, Neptulon being the other. Ragnaros is quick to anger, and his goal is raw power. Ragnaros doesn't do politics. Ragnaros does slavery and destruction. This slavery is apparent in Blackrock Mountain, his current seat of power in Azeroth. Emperor Thaurissan of the Dark Iron Dwarves summoned Ragnaros to Azeroth in an attempt to win the War of the Three Hammers, the war that set the Dark Iron against the Wildhammer and Ironforge Dwarves. It did end the war, but not in a win. Ragnaros was summoned, destroyed most of the Redridge Mountains, and his arrival was heralded by the creation of a destructive volcano. Ragnaros and his lieutenants proceeded to beat, break, and enslave the Dark Iron Nation.
Ragnaros was believed to be defeated in World of Warcraft, prior to The Burning Crusade, but it seems that isn't the case. Neptulon is still eagerly at war with him as stated in the Midsummer Fire Festival quests. He is, apparently, still alive and well in Blackrock. Unusual for certain, but they've given Onyxia life again in the Warcraft comic, despite being revealed for what she is and killed in WoW as well.
Neptulon the Tidehunter
Neptulon is much like Ragnaros in that he is one of the militant Elemental Lords. Neptulon is regarded as the most powerful of the Lords, and the amount of activity we've seen from him on Azeroth and Outland supports this. Neptulon isn't above manipulating and using mortals to further his goals, or even working with them. If you've ever done Molten Core, it is through Duke Hydraxis, an agent of Neptulon, that we defeat Ragnaros.
It has been a longstanding theory that Neptulon is the being that transformed the Highborne into Naga after the Sundering, but recent plotpoints may disagree with that. The faction of Naga that support the Will of Neptulon are labeled heretics. Skar'this in Coilfang is an example of this. It is possible Neptulon did this for the Highborne, but there's a little more evidence to the contrary now.
Currently, Neptulon is working with the Twilight's Hammer to summon one of his lieutenants, Lord Ahune, into the realm of mortals via Outland. From there, they will escort Ahune to Azeroth to wage war with Ragnaros. The Twilight's Hammer seek to usher in the apocalypse, but it is more likely that Neptulon's aim is simply to take out Ragnaros and further the realm of water. That may unintentionally usher in an apocalypse, but it isn't in his nature to do that specifically.
Maws, who bears a piece of the Sceptre of the Shifting Sands, is another example of Neptulon having an active hand in the world of mortals.
In summation, the Elemental Lords may come across as evil or malicious, but overall they are just an embodiment of nature. They are chaotic and interact in a way that the elements would in our world if they were massive behemoths with the strength of demigods and gained sentience. That's just the nature of... uh, nature.