As for Kreator keeping it true - some would argue.
I personally can't blame Mille for experimenting, even if some of the gothic and industrial paths he'd chosen to explore led nowhere. They eventually came back to their thrashier style with some silly little gothic melodies (when Thrash was big again), but it's not half as good as Extreme Aggression or Coma of Souls.
Some of their music I do find enjoyable and listen to it from time to time (different eras), but they don't get above a certain level of simplicity of the music and do get repetitive pretty often which is boring. As for German Thrash I always preferred late DeathRow (+ End Amen), Mekong Delta, Vendetta, Paradox, maybe even Tankard and Exumer. Still, a lot of Kreator's classics are very headbangable, even if not terribly original. Plus, they've managed to stay an active band like Testament, Overkill, Annihilator, Tankard and very few others outside the Big 4.