WARNING: The following contains spoilers for Marvel's Journey to Star Wars: The Rise of Skywalker -- Allegiance #2, by Ethan Sacks, Luke Ross, VC's Clayton Cowles and Lee Loughridge, on sale now.
Kylo Ren isn't exactly the most stable of individuals. Even within the evil First Order, he's been established as having a short temper and even shorter patience. All of this (coupled with his desperation to wipe out the Resistance once and for all) has led him to become increasingly more violent and fearsome, and he's trying to make the First Order just as ruthless.
However, along the way, he suggests that the Empire was too "lenient" for its own good. Which might be a bit of an overstatement, given the history of the Empire.
Mercy Is For The Weak
The comic follows the events of The Last Jedi, so the Resistance has largely been wiped out by attacks from the First Order. This has left them with almost no weapons or supplies, forcing the remaining Resistance members into desperate situations to try and gain new tools for their war against the First Order. They've stopped at various locations, including a shipyard at Fondor. The citizens of Fondor refused to help the Resistance and turned them away, giving them no weapons.
Yet, when the First Order arrived, Supreme Leader Kylo Ren reveals that even communicating with the enemies of the First Order is considered an act of treason. On top of taking control of the shipyards for themselves, Ren personally executes the head of the station via beheading. Before his death, the representative tries to argue with Kylo, claiming that not even the Empire had forced the shipyard into their employ. Instead it recognized some level of freedom for the system and the people who live there.
Kylo Ren responds that this was because the Empire was too "leniant" in the application of its authority. He tells his men to spread the news of what happened on Fondor, and how this proves the First Order is willing to be harsher than ever before in their pursuit of the Resistance.
Let The Past Die
Kylo Ren has become increasingly dangerous, along with the First Order itself. He trained under Snoke, the First Order's enigmatic leader who taught him the ways of the Force. Kylo Ren's also proven to be more unstable than anyone really expected, even turning on and murdering Snoke in The Last Jedi. That Kylo would decide to use those same efficient but violent tactics on a grander scale makes sense from a character standpoint.
But Kylo might be underselling the Empire a little bit with his comparison. Although the Imperial Forces weren't as quick to battle as the First Order, they were still established as power-hungry and despotic. Resistance was usually met with swift execution. They were able to go to war with ease, solidly holding control over the entire galaxy with military might. They were also the ones who designed the Death Star in the first place, using it to target planets and wipe out entire populations with a single blast. That's not even considering the litany of atrocities they committed outside of the movies in the various shows, comics and games set within the Star Wars universe.
Kylo seems to be focusing on the brutal tactics of his idol, Darth Vader. Vader would likewise dispatch his allies as quickly as his enemies if they so much as briefly disappointed him. So if Kylo is still doing his best to be the new champion of the Dark Side and carve out his own grisly path to victory, it makes sense that he would try to become more ruthless as a result.
If everyone in the Empire had acted like Vader, they certainly would have been more ruthless -- but they probably wouldn't have maintained enough people to construct the Death Star, which gave them perhaps their most powerful weapon and allowed them to enforce their power over entire worlds at a time. This commitment to merciless ideals might be the beginning of the end for Kylo's rule...