Obsession is at the root of many Star Wars stories. Darth Vader's own rise and fall could be attributed to his all-consuming anger harnessed by the Emperor. In turn, Kylo Ren was obsessed with Grandfather Vader/Skywalker, idolizing him and emulating his single-mindedness.
It's a trait that seems present in several characters who seek to serve the Emperor, including General Karbin. Appearing in Star Wars: Darth Vader #6, Karbin was a Mon Calamari commander who served during the Clone Wars. He was severely injured during the conflict, spending almost two decades on life support until he was rebuilt with a fully cybernetic body inspired by General Grievous' own technological enhancements.
In the comic, the cybrnetically-enhanced scientist Cylo has enhanced himself to the point that he is basically an immortal system. His mind lives on, transferring to new cybernetic bodies whenever necessary. He is retained by the Emperor to create Sith- and Jedi-like warriors who can emulate Force talents, but aren't actually Force-sensitive. Ultimately, these warriors are designed and trained to replace the likes of Darth Vader himself.
Karbin is one of these creatures, whose enhancements are very similar to those of General Grievous, with four arms and sabers included. Many of Cylo's cybernetic candidates are formidable, but few are as obsessive about rising through the ranks and supplanting Vader than Karbin himself.
He appears sporadically throughout Star Wars: Darth Vader, single-mindedly vying for a new command. Karbin is even tasked with finding the elusive pilot who destroyed the Death Star, which puts him firmly within Vader's sights.
At this point in the comics arc, Vader has discovered that he has a son, Luke, and that his son took down the Death Star. Darth Vader doesn't want Karbin to succeed in his mission, because he wants to find Luke himself. Karbin is aware that Vader is also looking for the pilot -- despite orders to the contrary -- and obsessively dogs his efforts, wanting to foil Vader's plans just as much as the Sith Lord wants to stymie Karbin's. Vader is constantly working behind the scenes, compulsively second-guessing what the Emperor has in store for him, trying to solidify his position in the Imperial hierarchy, all the while trying to keep his newly discovered progeny secret.
Ultimately, each character's all-consuming antics come to a head, with Vader and Karbin facing off in a lightsaber battle whose victor is never in question. Full of bluster and bravado, Karbin assumes he will best Vader, but he is vanquished handily in the end.
While Darth Vader seems secure in his role as Emperor Palpatine's right hand, the truth is that there are always replacements on deck. No one is safe. Because Karbin assumes he'll be next in line, he does whatever he has to do in order to supplant Vader. However, how single-mindedness and his overconfidence lead to his downfall.
Similarly, Kylo Ren fell to the same fate. In his obsessive climb to measure up to his grandfather, the Sith-in-training constantly came short of that goal, ironically following in the Sith Lord's footsteps by choosing to ultimately side with the light and thwart the Emperor himself, to his own peril.
It was a dangerous thing to be part of Star Wars' Galactic Empire, the New Order, or whatever regime the Sith presided over. It is a universe and history chock full of sup-plantation, where, despite obsessive and fanatical drive, no one's role (or life, for that matter) was safe and everyone was seemingly one step away from being choked to death by the Force, assassinated, or -- if they were lucky -- demoted.