Slayer Kreator and Destruction all have distinct simularities. When you have bands playing the same style and instruments the same way obvious distinctions are going to be made. I really doubt any riffs are intended on being ripped off but when you play a guitar,etc.. a certain way and have the same influences things are going to sound similiar when you are striving on the same concept. Two bands of a same genre could not even know each other exist and have simulaties. It's not like one band is jazz and one band is blues. They are metal bands with distorted guitars with the same music concept with the same influences. It's not rocket science and instruments are generally played the same way. You can't defy the person who creates an instrument.