Massacre
This issue of Uncanny X-Men takes place right at the beginning of the Mutant Massacre. This is a dark chapter in a dark story arc. This is a tight, well paced issue with some lasting implications. This story sets the pace for how the Mutant Massacre as a whole will affect the X-Men and their companions.
Chris Claremont creates very evil characters in the Marauders, a group of mutants hell bent on killing all other mutants. They have no redeeming qualities and at a point you want the X-Men to kill the Marauders because they deserve it, but the X-Men are not all meant to be killers. This story shows the X-Men failing at the one thing they should be doing well, protecting those that can't protect themselves.
The penciling duties are split between John Romita Jr. and Bret Blevins, by todays standards it's a little simplistic perhaps, but I think it presents the story well, especially when Colossus snaps.