Second, how I would group them.
1 Kill 'Em All - Mustain-era
2 Ride the Lightning - transition between Mustain-era to Burton-era
3a Master of Puppets - matured Burton-era
3b...And Justice for All - Burton-era, but wanted a different sounding, cancelled the bass
4a The Black Album - the rise of Newsted-era, I believe he added way more to the songs the credits lead us believe, I think he loved Metallica and he said: "do whatever with my stuff", and the others claimed it (I do not believe they did it with purposeful malice or soemthing like that, but they exploited his willingness and kindness, and when later he tried to make something on his own this led to the break, because they shat themselves he won't give them more), I also think the others wrote lot of stuff too (compared to Kill 'Em All for example)
4b Load - prime Newsted-era
4c Reload - the low of Newsted-era
5a St. Anger - "wtf we're gonna do alone"-era, reaching back for inspiration to the Mustain and Burton eras, but incorporating elements of the popular trends of those years (nu-metal n shiet)
5b Death Magnetic - ditto, reaching back even more for inspiration, starting to look more similar to the old days
5c Hardwired... to Self-Destruct - ditto, but now they arrived far enough to recycle chunks from the first couple of albums (like fashion repeating itself at every three decade)