I'm on Iron Maiden > Killers camp as well.
The debut was not a band starting out. It was the culmination of 4 years of playing, doing early tours, Steve Harris having to find members who could fit the vision he had, learning the craft, going through the grind. There was no "put your song on YouTube and it becomes a hit" back then, they had to do the work, the gigs, the word of mouth, taking around the cassettes, all of that. The debut album was four years in the making, and it was nearly a greatest hits of all the songs that came to be in that period of time. Killers had to be done in a much shorter time frame, and while there's nothing specifically bad on it (hey, Adrian Smith is on there, for starters), I don't think it holds up to the predecessor. Also, Paul Di'Anno could take the band any further and it was the right to make a change, and boy what a change it was.