"Thrash metal" (originally called "speed metal" by many) employs most trademarks of classic heavy metal, but it combines them with a punk ethos and a generally harder edge. This can be seen in spades on Kill 'Em All. Songs like "Hit the Lights" and "Motorbreath" are practically heavied-out hardcore tunes with more technical (but no less frantic) solos. "The Four Horsemen" and "Seek and Destroy" are the longest songs on the album, each featuring changes in tempo and mood, but they are as tough-sounding as anything else here. Also important is "Whiplash", as the band is referred to in the lyrics. By name-checking themselves, Metallica became more self-aware and thus more dangerous-sounding, which is essentially what thrash is all about, really.
Four out of the ten songs here were co-written by original lead guitarist Dave Mustaine (whose band Megadeth would go on to rerecord "The Four Horsemen" under its original title "Mechanix"). This is actually more of a big deal than it might seem, because it gives this album a looser, punkier feel more similar to early Megadeth than all of the Metallica albums to follow. This reliance on Mustaine's material makes Kill 'Em All an imperfect album, but even if it doesn't necessarily "define" thrash metal, it was still the first thrash album ever, and there's something to be said for being first. Slayer and Anthrax would follow suit within a year, but debuts by other vital thrash bands like Kirk Hammett's alma mater Exodus and the aforementioned Megadeth wouldn't come out until a bit later, and many others even later than that. This proves that Kill 'Em All paved the road for future thrashers more so than any other album. Still, it would not be held in as high regard if it was not such a furious display of technical aggression that did not contain a single weak track. Rest assured, that Kill 'Em All is and remains over 30 years later.