Wonder Woman might be an Amazonian warrior, but she's never been a huge fan of killing. If the occasion calls for it, then yeah, you can expect to fall on the wrong end of her sword, but it's not a decision Diana has ever taken lightly. Case in point: Maxwell Lord.
Lord, in every Earth, universe, timeline or otherwise, is a nasty piece of work. A former bankroller for the Justice League, it was revealed in Geoff Johns' Infinite Crisis that the real reason behind Lord's supposedly altruistic behaviour was to actually collect sensitive information on the world's superheroes. When the original Blue Beetle, Ted Kord, confronted him about this, he shot him, thus killing one of the DCU's most beloved superheroes.
Little did he know at the time, however, that it would also lead to his own death. After tasking an army of OMACs to hunt down every last superhero out there, Lord takes control of Superman's mind (somehow), and manipulates him to attack Batman, with Supes seeing him as a version of Braniac and not as the world's greatest detective.
At odds at what to do, Diana rushes to Lord's location and demands that he relinquish control of the Man of Steel. With her lasso around him, Lord replies that all she can do is kill him, lest she witness two of her partners kill each other in public view. With no other option presented, Diana breaks Lord's neck, killing a twenty year-old character and straining her relationship with the League and the public in the process.
Infinite Crisis isn't one of DC's finest reads, but there was no way back for Lord in any case, unless you count his rebooted appearances in the New 52.