Leto (as mentioned before) used their own physical strength rather than use Zaleth’s powers two vital reasons:
Leto doesn’t consider Zaleth’s powers their own and they weren’t looking to use borrowed powers to settle things with Killian. If the demon did come and aid them in the fight, Leto wouldn’t really feel like they handled the situation; they’d feel like they relied on someone else to settle the matter for them. Once Killian threatened their family, the matter got extremely personal and they weren’t about to rely on someone else to handle a situation that they felt was one they needed to settle by themselves once and for all. Even if a close friend of theirs like Keano, Mabelin, or Rigel had been the one ready to step forward to help, Leto would’ve told them to stand back; in that moment, they were VERY determined to handle Killian on their own.
Even if Leto didn’t have the strength to hold their own in a fight against Killian, they would then prefer to give a verbal beat-down solely for the fact that they don’t trust Zaleth in any way to ask for his powers. They’re always have conflicting views with the little monster hanging out in their head for who-knows-what-reason and Zaleth makes it pretty clear that he isn’t fond of Leto’s quiet, altruistic nature, clearly reacting either disrespectfully or mockingly toward the young Light Mage. So even if Leto was more inclined to ask for help with handling Killian, Zaleth would be the LAST person they’d go to for that matter.
Also, Leto is the child of a former guardsman (and is a guard themselves); it just felt really natural (and kind of cool) to imagine that at that moment in the fight, Leto showed Killian the same amount of protective ferocity that their Father did when Killian came to harass Leto at the General store (though I kind of picture the Father’s ferocity to be more like a bear’s where Leto’s ferocity is much more akin to a black panther’s). The Father and Leto certainly don’t always see eye-to-eye due to Leto not really keen on their old man’s jealous attitude toward their magic (and also due to them both having relatively different temperaments). But when all is said and done, Leto does love their father and was quick to tell Killian ‘their father was twice the man he would ever be, no matter his faults’. So far, I see Leto’s relationship with their Father being a occasional headbutt due to opposing views but, more often than not, the two are the type who would sit silently with one another with very little words and genuinely enjoy the moment due to having a strong bond at the end of the day.
…I know the MC is in their 20s, but I can’t help picturing if they have a job with the RO and they came to meet them at their house (and weren’t Mabelin), that the Father would take one long, hard look at them, close the door behind him and stand his full height before the RO and be like “Any particular reason you came for my daughter/son/child ”. I’m pretty sure it won’t happen (MC seems past the age when parents worry about what type of relationship the person they have yet to meet before has with their child), but it’s a little amusing to imagine how the current ROs would handle the situation when confronted with a slightly protective father.