Once you've made Reign In Blood where else is there to go? Every thrash/black metal band on the planet has tried to match it and none have, it is the all time number one. Even Slayer couldn't top it, they didn't even try. I think they realised it was impossible so they slowed down and came out with SOH which is a damn good album. Everything Slayer did up to Decade of Aggression was fucking great. After that, meh. Their concerts were always excellent but like all the other bands of that period their albums from the 90s onwards were average at best.