Sodom. Even when they were doing their more punkish style, it was only a reflection of Tom's influences-he alwas had a few punkish songs in his first albums. So if you want one that has stayed "true" more or less, take a listen to Sodom (Tapping the Vein was what, 1992? Incredible thrash album.)
I think Thrash is a very exciting style, but many people are pre-conditioned to hate it. That's fine, because it's your choice. That you only like the slower Kill'em All songs show how you really will (probably) never get into Thrash (nor Speed Metal). Hit the Lights, as short and simple as it is, it's amazing. I remember listening to it over and over again on LP as a kid. Mike Sifringer from Destruction was really inspired by that song, and I can see why. Phantom Lord, Metal Militia, all amazing. Staying so-called "true" was hard to do to keep going in the "music business", because not even after the relative comercial success of Thrash Metal with the so-called "Big 4" was Thrash really a socially accepted genre (the "4" were not always the best standard-bearers for Thrash, IMHO.)
So, it's not that Thrash had to evolve (I.E be dumbed/slowed down), but that most people for unknown reasons just really hated the genre, and it was never meant to be a popular genre. I am actually surprised that the sons of Thrash actually did relatively better (black, death, etc.), because (of course) I still prefer just good ol' Speed/Thrash, and it's not as if those genres are any more palatable to the general public.
I don't have much respect for the Black Album myself, but I don't care whether somebody likes it-to each their own. Indeed, Metallica was never the fastest or more brutal Thrash band. But they still had Thrash songs in all of their first 4 albums (of course, not all, but some of these songs were very good even when they weren't Thrash.) Ironically, it is the hit from AJFA that I don't like as much, although it is a good song (One). The rest of the songs are pretty amazing and it's rather sad (Thrash metal-wise, not economically-wise for them) that they opted for a more commercial sound-especially since in their initial years they were all about being "true". I don't think it was evolution, but just an economical decision (I mean no offense-it's ok for them to do whatever they want, although it made it harder for the other bands that were riding on Thrash's relative success.) So, yes, I don't like TBA, but I don't care if you adore it, as who I am to tell you what to listen to?