I mentioned in my post last night how Stefan and Klaus are so similar in the 1920s. Both have father issues, fathers who saw them as abominations. What's more, while Klaus is literally running from his father, Stefan's running from his father, too, by diving into his Ripper days. That's what getting lost in the bloodlust is about. That's where it came from in the beginning. Drinking from his father. Both Stefan and Klaus experience these impulsive rampages brought on by the extreme rejections of their fathers. Stefan's Ripperdom is rebellion from his father's control, his father's judgment. It's an escape, a means of running from a ghost whose approval he can never win. So he has to become the monster his father believed him to be. That's the first step. Then enters Lexi, watching Stefan diving into his kills as a self-fulfilling prophecy, reliving the extremes of bloodlust and guilt that he felt when he killed his father. When Stefan's reliving the kills of his victims, he's forever reliving how he killed his father -- it's a form of self-torture, feeding this neverending cycle of guilt. And Lexi just forces more trauma onto Stefan as a means of controlling him, since she sees pain as the only way to "break through" to his humanity, not realizing that he's a Ripper because he feels too much and he can't find a way to make the pain stop.
Stefan can't fight Lexi, can't defeat her and win back his control through self-acceptance because, as she reminds him in "Ghost World", she's "dead" just like his father*. He can't escape the closet full of all his kills where he punishes himself over and over again, reliving the kills, not out of pleasure, but out of self-punishment (why else would he put his father's name on the wall?). He can't escape the dungeon where Lexi stabs him over and over again to make him feel a humanity which was never lost. He can only let Lexi wear him down, beat him down, until he's weak and under her control -- it's the violent form of vampire compulsion, compulsion through domination and torture.
*This pattern of control through torture is repeating with Caroline and her father, too. You can't force someone to feel a humanity that they'd never lost. Caroline does have control. However, the method of facing one's fears to find control does find success when Tyler submits to it willingly.
Klaus, being an Original, can control Stefan even more immediately where it takes Lexi years to become the Master of Stefan. But with Klaus, Stefan can fight back for control, one step at a time, motivated by love for his brother -- love for Damon causes him to return to submit to Klaus, love for Damon helps him fight his way back to humanity. And in defeating Klaus, Stefan attempts to ultimately free himself from the extremes of his loss of control and his desperate need to control himself (the way Klaus has to dagger his family to keep them in line, the way Lexi has to stab Stefan to get him to feel, to break through). But traumatic torture isn't the way to reclaim humanity; like Katherine said, you have to "let it" in.
Sidebar: How much do I love that no one could see that the humanity that would endanger the plan to kill Klaus in "Homecoming" came from Katherine and Stefan, the two people who everyone believed HAD no humanity to weaken them. Like Lexi being blind to Stefan's humanity in his darkest hours, everyone discounts the humanity in Katherine and Stefan.
The real cause, of course, being that Stefan can never forgive himself for murdering his father and every time he feels bloodlust, he's taken back to the first time he felt that way, feeding from his father as he lay dying before him and calling him a monster. It's PTSD, being forced to relive the worst trauma of his life, and human blood is the trigger. That's why drinking human blood involves so much personal shame for Stefan, whereas Damon can enjoy it because he associates drinking blood with sexual pleasure and intimacy, thanks to Katherine.
Here's the kicker, when Klaus compels Stefan to "TURN IT OFF", turning off Stefan's humanity allows him to free himself from the self-hating cycle. Shutting down Stefan's humanity means shutting down the emotional triggers of Stefan's PTSD. That's why Stefan's not Ripping and then putting the bodies back together while under Klaus' compulsion, because being a Ripper is about Stefan's traumatized human feelings and his enormous guilt. Thanks to Klaus' order to "TURN IT OFF", Stefan can drink blood without associating it with his feelings about his father. And this disassociation is a break in the PTSD cycle. So, there's actually some hope for Stefan to not return to his Ripper days thanks to Klaus breaking the cycle. And how ironic, that Klaus uses compulsion to turn off Stefan's humanity to turn him into Ripper, whereas Lexi used compulsion in the form of torture to stop Stefan from being Ripper -- only Lexi's method only keeps Stefan from facing the source of his loss of control, the trauma, whereas Klaus' method actually helps him gain the distance to stop reliving the trauma.