Max was not the first person Diana has killed. Not by a long shot. In fact, he's not even the first human she's killed. Hell, even if you take Asquith and Circe's bestiamorphs off the list for being nonhuman at the time of death, there's still a fair sprinkling of people through Perez and WML's runs who you could conclude she killed without stretching what's on the page. Certainly she's been at war more than once, and was killing sentient beings during the khund invasion and the pirate revolution against the sangtee.
She's the S_D of the DCU. Everybody knew it was happening, but until one jerk made it impossible for anyone to keep pretending they didn't, they were all willing to let it happen because they knew it wasn't the evil that the party line presented it to be.