I actually find selfish self-insert fics to be a fascinating pressure cooker for character conflict, specifically because they short-circuit the usual "found family" narrative. When the SI is openly out for their own survival or gain, it forces the canon characters to react to an unpredictable element. It's less about the SI's internal conflict and more about how their presence warps the dynamics between the original cast.
Take a 'Harry Potter' SI who decides to hoard knowledge or artifacts for themselves. The central conflict isn't just between them and Voldemort; it's between them and Dumbledore, or Hermione, who would see that selfish hoarding as a moral failing and a tactical disaster. The story becomes about trust, or the severe lack of it. The SI's selfishness can expose hypocrisies in "good" characters—like, would the Order really welcome anyone, or only those who perfectly align with their martyr complex?
These fics often shine when the SI's selfish goal is mundane, like just wanting a quiet life, but their methods to secure it (allying with questionable people, withholding warnings) create massive external conflicts. The tension isn't necessarily about good vs. evil, but about different, competing self-interests clashing.