• He isn't very nice, to say the least. To say the most, he's selfish, petty, rude, manipulative, violent, narcissistic (just look at his friggin' name) sociopathic, psychopathic, and megalomaniacal.
• While he does openly make his employees' lives a living hell and scientists such as Dr. Samuels are only working for them due to him threatening their loved ones,
• Keeps his daughter Angel locked up and pumped full of Eridium because her Siren powers can power the Vault key. She's been that way for so long that cutting the Eridium flow would kill her, so she literally has no release but death from her torment.
• While it's much more subtle than other examples in the series, Jack is still very volatile, capricious, sadistic, and generally mentally unstable,
• He also used his own employees as punching bags, to the point that Vasquez thought he had a special standing with Jack because Jack hit him every time they met.
• Representing pride, greed and eventually wrath as he undergoes a Villainous Breakdown, as well as having the storybook traits of being rich, bullying and cowardly.
• His dictatorship of Pandora is extremely authoritarian, he has posters with his face plastered everywhere, lots of people, and all Hyperion personnel stationed there either see him as a near-godlike figure or an outright god, and he is willing to torture anyone who opposes Hyperion.
• Of the "Truth Twister" variety. He has no trouble misleading, telling the part of the truth he wants you to hear, or telling his own incredibly biased version of the truth.
• According to one of his rants, Jack apparently scooped out someone's eyeballs while their kids watched after he tried attacking Jack with a spoon.
• You collect some ECHO recordings in Liar's Berg that describes him confronting Helena Pierce, making fun of her appearance, shooting her in the head, laughing at how bloody hilarious it was, before ordering the massacre of a trainload of civilians she was trying to take to Sanctuary.
• He has no real understanding of the concept of "good". In his eyes, a hero is something you can just say you are and enforce using the right words.
• For all his altruistic motives, Jack still manages to be the biggest bastard on a planet loaded with unrepentant mercenaries, thieves, lunatics, slavers, monsters, pirates, sadists, grifters, and cannibals.
• When this is pointed out to him by an employee, Jack hints that he'll have said employee's children killed.
• He's well-spoken, energetic and charismatic, but he's also a petty, sociopathic monster who does nothing but verbally abuse you in conversations. And his composure doesn't just break, it shatters into powder.
• After becoming Handsome Jack, he does pretty much whatever he feels like, reasoning that he's a hero, and therefore anything he does (no matter how twisted or depraved) is heroic.
• In fact, he sees the whole world as a story being told with him as the protagonist, incapable of doing any wrong.
• Handsome Jack believes he is the hero of the story no matter how much of an asshole he is, and believes all of the atrocities he has committed are simple Shoot the Dog moments.
• Such as when he tortures Tannis, but lets her live, thinking he's being "merciful."
• He shoots Helena Pierce off-screen, mutilates and kills Bloodwing, and then kills Roland. All essentially in cold blood.
• Jerkass: And that's the nicest way to describe him. Other ways used in-game: douche, tool, asshole, fascist, sociopath, maniac... The jerkassery was a pre-existing condition, but it used to be tempered by rationality. Now it...isn't.
• Then there's his murder of Bloodwing right in front of Mordecai, followed by a hilariously badly played violin, which was meant to be played in a sarcasticly sad manner. Even afterwards, he puts what's left of her mutated body on display in Opportunity.
• If the player chooses to kill Sheriff Nisha (who happens to be Jack's girlfriend) in Lynchwood before completing the game, while Jack is a tiny bit miffed that she's dead, he's much more surprised that the Vault Hunters actually managed to anger him at all, showing just how little he cared for his own girlfriend.
• In Opportunity, one of the ECHO logs you can find details that he murdered one of his employee's children simply for pointing out flaws in Jack's plan to build Opportunity.
• In one ECHO call to the player, he explains how he and Wilhelm tortured Tannis by beating her for hours and then taking the Vault Key from her, only sparing her because "that's what heroes do. They're merciful."
• According to the ancillary material, he sent Sanctuary a box with the heads of Pierce and Roland, and what was left of Bloodwing's.
• He's mentioned to have done horrific slag mutation experiments against Pandoran citizens in the Wildlife Exploitation Preserve, burned down New Haven, massacred whole towns as his army swept across Pandora and launched a 3-year-war against Sanctuary and the Raiders...but we don't get to see any of it, and only hear it in ECHO logs.
• In addition to enslaving Angel as a child, he's not above the outright murder of children — Sir Hammerlock mentions so, and an ECHO recording heavily implies that he had a Hyperion colleague's children killed for pointing out that Opportunity is Fascist, but Inefficient. Also, if you listen to Helena Pierce's echo logs, you can clearly hear women and children screaming when Jack orders Wilhelm to open fire.
• All of this said, he's still an insane murderous douchebag, as is noted by basically every last one of the protagonists at one point or another.
• However, Episode 5 reveals that all of it was likely a ruse and once he's out of Rhys' head he has no problems with having him killed, either out of rage or as his own idea of a "reward".
• In the final episode, after jacking back into Rhys' mind, he makes Rhys strangle himself with his own arm, forcing Rhys to pry the arm off.
• His mentoring of Rhys is really him just trying to convince Rhys to do what Jack wants, which is mostly handing Jack more power, either over Rhys' body, or Helios itself. Of course, Jack doesn't see it that way. If you were buddies with him the whole time, he takes Rhys's unwillingness to become the very first Robo-Jack pretty hard, as if he doesn't understand how anyone could not want to be killed and have a robotic endoskeleton shoved into their corpse for Jack's AI to inhabit.
• While he's still a hateful digital lump of narcissism, recalcitrance, viciousness, and vice, he's much more eloquent about it and can do more than just stomp around and talk about how awesome his nonexistent abs are.
• Keep in mind that Jack had Angel imprisoned, enslaved, wired into a computer network and used her both as his personal databank and to charge a Vault Key using her Siren powers, a situation that she chose to escape by killing herself.

(all quotes from tvtropes.org summarizing why i hate him)