Dwight(the guy who took the Bow) is sent by his boss with other men to kill one of Rick's group as revenge for Rick killing a small group that tried to attack him on the road bar one that got away.
Abraham and Eugene are outside the walls when suddenly a bow from Dwight kills Abraham and Eugene is kidnapped so they can get inside the walls. It's what triggers the all out war between Rick's group and The Saviours. It was controversial at the time, because Abraham was pretty much Superman in the comics so it felt weak for him to die so suddenly and Kirkman said it was the only thing in the comic he regretted...although he still liked the idea of the death and that not everyone can go out a hero.