For 7 weeks of Marvel's Inhumans' 8 week run, the series has puzzled fans as to why Maximus (Iwan Rheon) set his plan to overthrow the Royal Family of Attilan into motion. Because the Inhumans' society is built upon a caste system, Maximus' calls for revolution and equality styled him as a hero of sorts, even though his methodology involves copious amounts of deceit and murder. The penultimate episode of Inhumans, 'Havoc in the Hidden Land', finally revealed what Maximus' plan is really all about and why he did everything he did. It wasn't to better Inhumans society so much as to 'better' himself.
From Agents of S.H.I.E.L.D.'s second season to present day, new Inhumans have been popping up all over Earth, which hasn't gone unnoticed by the Inhumans on the moon. King Black Bolt (Anson Mount) began a process of bringing some of the new Inhumans who are being born on Earth and being hunted by humans to the safety of Attilan by sending Triton (Mike Moh) to find them (though the series hasn't shown any of those new Inhumans on Attilan, so it seems like Black Bolt sent Triton on his first mission in the first scene of the pilot and Triton immediately failed and was killed).
Meanwhile, we learned over the course of the series that Maximus has himself been in contact with a geneticist on Earth named Dr. Evan Declan (Henry Ian Cusick). Maximus has been funding Declan's research into these new Inhumans. (Though with what funds, the show hasn't said. Do the Inhumans use any type of Earth currency?) Declan's job was to get DNA samples of the new Inhumans and catalog their powers. How Maximus eventually planned to physically acquire Declan's research is another question, but conveniently, the answer was provided by Declan helping break Black Bolt out of prison to study him, and circumstances ultimately leading to the Royal Family reuniting on Oahu and bringing Declan with them when they returned to Attilan to face the usurper Maximus.
In 'Havoc in the Hidden Land', Crystal (Isabelle Cornish) teleported back to Attilan with Lockjaw to deliver terms of parley to Maximus. Meanwhile, the rest of the Royal Family returned to the moon and made as their base a heretofore unmentioned Royal Bunker (basically a fallout shelter only Black Bolt knew about) to plan a war strategy in the event that Maximus wouldn't honor the parley, which of course, he didn't. The parley was a trade: Black Bolt would offer Maximus his human ally Declan in exchange for him standing down so that Black Bolt and Queen Medusa (Serinda Swan) could return to their rightful role as king and queen. The Royal Family would not enact any punishment on Maximus for his coup, and would allow Declan to give him what he wanted all along and what all of this was truly about: for Maximus to undergo a second Terrigenesis.
As Maximus explained to Declan, Terrigenesis is the fundamental aspect of Inhumans society and is more or less a religious experience. For an Inhuman to undergo Terrigenesis and not gain powers - to be "ignored" by Terrigenesis - is the gravest insult, which is exactly how Maximus felt when Terrigenesis denied him powers and turned him into a normal human. What Maximus plans for himself is to undergo a second, manipulated Terrigenesis where he could choose what powers he would gain from the process. However, the Genetic Council would have forbidden this because undergoing a second Terrigenesis could lead to untold psychological defects, but Maximus isn't concerned with any side effects. (Let's face it, Maximus is already afflicted with psychological issues.) Since the Genetic Council would have stood in his way - and indeed they tried to revolt against his rule - Maximus had them killed.
As for his coup against his brother and family, that was fueled by several desires. His plan for a second Terrigenesis was one aspect of it - Black Bolt would also have forbidden Maximus' attempt to gain Inhuman powers he was already denied according to their society's laws. Jealousy of his older brother and an inferiority complex provided Maximus' other rationale for turning on his family. Maximus probably does want to go to Earth and probably also truly believes the Inhumans should leave isolation and take their place on their homeworld, but he used this sentiment to gain favor with the lower castes of Inhuman society. He fanned the flames of equality and painted himself as someone who can show them how to rise up from the very bottom - rejected by Terrigenesis - and become a king. Being able to control Terrigenesis and its results is another desire of his, but it's unclear whether he would share this process with the lower classes.
However, Maximus is also a nihilist, which he proved when he betrayed the rules of parley, took Declan, and refused to surrender the throne. Then the Royal Family played their hand and engineered an attack on Maximus. Though Gorgon (Eme Ikwuakor) perished in the previous episode, it turns out Triton was alive all this time and returned to Attilan with the Royals. Triton killed Maximus' personal guards and trapped him in the Royal Bunker to come face to face with Black Bolt once more. This is when Maximus revealed he enacted a fail safe: if he dies, the protective dome surrounding Attilan would be destroyed, exposing everyone in the city to the vacuum of space and killing all of the Inhumans. The episode ended in a stalemate between the two brothers to be concluded in the season (possibly series) finale.
As for Gorgon, his death lasted all of one episode. When Karnak (Ken Leung) realized Maximus' plan for second Terrigenesis, he decided to use it to resurrect his cousin Gorgon. Karnak recruited Auran (Sonya Balmores), who has healing powers and grew to have second thoughts about casting her lot with Maximus, to his side. Injecting Gorgon's corpse with Auran's DNA, Karnak placed Gorgon in the Terrigenesis chamber. Ultimately, it worked and Gorgon came back to life, but seemingly as a mindless monster chasing Declan, who happened to be in the Terrigenesis room when Gorgon awoke. The side effects of second Terrigenesis looks to be how Inhumans introduces the concept of Alpha Primitives, which in Marvel Comics are the deformed, monstrous victims of failed Terrigenesis, into the series.
The preview for the finale already spoiled that the dome around Attilan shatters and contains the dialogue from Medusa that it was time for the Inhumans to return to Earth, so all that's left is to see exactly how everything shakes out next week and wait to hear if Inhumans somehow receives a second season order from ABC.
The season one finale of Marvel's Inhumans airs Friday, November 10th @ 9pm ET on ABC.