9. Daredevil Kills Bullseye
The Shadowland event is controversial for many Daredevil fans. On one hand, you have Daredevil being forced into taking control The Hand, an evil ninja organization, to try and turn them into protectors of New York City. But on the other, you have Daredevil giving a bunch of training killers license to go after the lowest level criminals of NYC, which inevitably leads to fights with the other street-level heroes of the area.
Daredevil broke two rules on this one: the no killing rule and the lesser-known "don't join your enemies" clause. Daredevil's origin since Frank Miller's The Man Without Fear has been tied to the ninja war between The Chaste and the Hand, so becoming the leader of the Hand is a bit of a no-no to Daredevil's mission statement.
As for the No Kill rule, Daredevil has always kept it a bit fast and loose, mainly sticking to it in the same way that Batman usually does: break all the bones but let them live. Except in this instance. Bullseye had already leveled a city block, kill over a hundred people, and then slaughtered a couple dozen of his men before a goading DD into a final confrontation.
Tainted by the darkness of the Hand, Daredevil breaks both of Bullseye's arms and shoves a sai through his chest, in a brilliant mirror of the now-iconic death of Elektra. He gave into his rage to break his golden rule.