I think Killers was inferior to the debut for quite a few reasons.
1: It lacked the clinching epic. The title track was great, and Murders and Wrathchild are fun, but they aren't really epics per say. Phantom's status as an epic is debatable, but what isn't is the fact that it's one of Maiden's finest. Maybe you could argue Prodigal Son, but that's a huge stretch.
2: A lot more average to mediocre content getting greenlighted like Innocent Exile, Another Life, Drifter and of course, The Ides Of March.
3: The production quality went up somewhat, but that kind of worked against the album, almost as if they were trying to meet halfway between refined and stripped-down. The Bruce-era went from refined (NOTB, POM) to very refined (Powerslave to Seventh Son).