Yes, it seems it is established that those words were in the lyrics of the song you heard. I stated that those words may not have been the actual name of the song. When you say you were "watching" their concert, do you mean you "watched" it by means of a video of it and not in person and this video of it displayed the title of the song on the screen and it was labeled as "Kill Them All" (written as "them" not "'em" like the album)?

edit: (sorry, missed that link and am checking it now to see if it can help me to help you find what you're looking for.)

That video is them playing Whiplash which is track 6 from the 1983 Kill 'em All album. If that video of them playing that song is footage from the actual concert that you watched live in Germany, then that helps narrow it down to early 80s when the song was played. That cuts out decades of possible songs.

I still have all their old albums on cassettes in a shoebox from growing up. I also have some live and unreleased garage and live stuff recorded on cassettes like them playing an early version of Devil's Dance before that song was released. It has been decades, but I don't recall any of the songs I have being a recording of a song called Kill Them All. I'll see if I can find a cassette player around here and give those old tracks a listen to see what all is on them.