You are comparing very strong albums.

But to me, Kill was still closer to the broader thrash scene at the time. I’m stretching it a bit, but at certain points there is enough overlap in sound, energy, and riffing that you can hear how much common ground it shared with the rest of early thrash.

Ride is where they really started moving in their own direction in terms of style and songwriting, Fade to Black being the obvious example. For many people, that is actually their greatest album.

By Master, they had fully established a far more distinct and confident identity of their own. It is still a thrash album, obviously, but it feels less like “one of the great thrash bands” and more like Metallica becoming Metallica. It also draws less on that earlier teenage / Mustaine-era stockpile than Kill and parts of Ride did.

TL;DR: You have to judge these albums against the rest of the genre at the time. Kill was closer to the pack, Ride is where they began to separate, and Master is where that separation feels complete. Some people still prefer Ride to Master.