History is full of examples of people who committed what we, and even at times, they themselves, believed were acts of pure evil simply because they believed that what they did was ultimately for the greater good. Throughout Angel's series, Angel has set himself up as the ultimate and final judge of what should be done, even when it changes the lives of people who he loves. His series itself is based on the idea that he he left the woman he loved, who loved him, because it was for the greater good. At no point did Angel sit down with Buffy over coffee and sweet rolls and say, "Okay, I realize that I'm so handsome, so sexy, and just so utterly irresistible that you will never be able to keep your hands off of me, and we already KNOW where that's gonna lead, so I think that we should really consider me moving back to Los Angeles. Are you going to eat the rest of that cinnamon roll?" He simplt took it upon himself to say, "After we squash Mayor Wilkins in his giant worm mode, I'm heading out for LA. Figure out a way to live with it."
Angel's decisions of that nature over the next five years steadily escalated to the point that, by the end of his series, he felt perfectly justified in ordering Lorne to murder Lindsey, who was at that time fighting on his side, because of what he MIGHT do in the future, assuming that they should survive what Angel himself described as a mere gesture, the attack on the Circle of the Black Thorn. Even further, he attacked the Circle KNOWING that it would result in the Senior Partners launching a full scale attack on mankind, and knowing that that attack would result in the deaths of untold numbers of men, women, and children. Yes, the SP's planned on attacking anyway, but at some time in the future, but the key phrase there is IN THE FUTURE, which meant that there was still time remaining to plan a better defense than half a dozen well intentioned if poorly motivated individuals staging a Kamikaze attack (except of course, for the fact that there would be no Season 6 in which such a plan could be carried out).
The point is, within the course of five years, Angel has progressed from making decisions for a single Slayer "for the greater good." to doing the same thing for the entire population of Los Angeles. Given the amount of time between that point and when Season 8 takes place, having Angel bring about as much loss of life as Twilight has done isn't such a stretch of the imagination, especially if he believes that, in doing so, he is actually saving the entire human race from an even greater danger, something like averting the "final battle" aluded to first in the Fray cycle and later in Season 8. There might even be some sort of tie in with the fact that Twilight's supposed purpose, the destruction of all supernatural forces and demons on earth, is exactly the result of the Final Battle itself.
