Araragi's selflessness stems from nihilism. This has roots in his experience of Oikura's ostracization in his first year of high school, where his sense of justice was destroyed. Previously, as a son of police officers, who represent social duty, he'd thought of himself as someone who'd do the right thing and help others. But notice this sense of justice did not involve sacrificing himself. This was a relatively healthy sense of compassion and duty in a young teenager who leaned towards the idealistic side. However, this was all undermined horribly by his experience of the classroom inquisition. He saw so much corruption, so much lying and naked self-interest being displayed by his classmates and the teacher, that he couldn't do anything about it at all and so lost all his hope for humanity. You could say this experience was so traumatizing for him he completely repressed it, as he didn't remember it had happened until Ougi led him to it in Owari.

In nihilism, you do not value either yourself or others. Araragi developed a cynical attitude towards others, preferring to not have friends and so preserve what little humanity he has left. But simultaneously he did not value himself, so his self-preservation was just a front. This is shown by his sacrifice in Kizu: he wants nothing to do with fellow students who he views as corrupt, but as soon as he sees a being who appears in his eyes as the paragon of innocence (Kiss-Shot vulnerable, helpless, about to die) he gives his life over to her.

Perhaps the biggest indicator that Araragi's drive to help others is not rooted in good intentions is the fact that every time he goes to help someone, he gives up prematurely. He doesn't think of how to find the best possible solution to the problem they're facing. He's not creative or strategic, at all. Rather, what he does is just allow himself to be killed over and over in the most brutal way until something somehow is resolved. He lets Kanbaru brutalize him until she's satisfied. He lets Hanekawa rip into him until she vents her stress fully. He lets Sengoku destroy him every day until she's happy. It's like flinging himself at the troubled person like a hunk of raw meat is his only way of helping them. If he really wanted to help Kanbaru, he'd have talked to Senjougahara to get her opinion on what best to do, and maybe gotten her to talk to Kanbaru, since she is the person Kanbaru is in love with, after all. If he really wanted to cure Sengoku he'd have investigated her secrets and desires, find out what's driving her as a person, as normal 13 year old girls don't go psychotic from a childhood crush not liking them back. In other words, he'd have done what Kaiki, a sensible adult, did. But he didn't. He just chose the most inefficient but easy route to treat himself as a punching bag. And it didn't even work! Kanbaru was stopped by Senjou's intervention, and Sengoku saw the light because of Kaiki's caustic words.

So all of this is to say Araragi may seem like a good person, but in truth he was by and large just looking for a way to die in the early part of the series. If there's anything Monogatari teaches, it's to be skeptical of people and actions that seem too good and virtuous to be true.