It’s not the cloning process that makes Angiers willing to kill. He’s set his mind on murdering his duplicate before the first time he uses the machine. The difference between his attitude at the beginning of the film, with the bird, and the end, when he’s got a hidden barn full of his own corpses, is meant to illustrate how depraved he has become in his obsession to beat Borden.
Yeah, I too was somewhat disappointed on my first viewing for just this reason. But once you get that playing with “real world magic” isn’t actually what the movie is about - it’s, as others have noted, a meditation on obsession - it works. Because Tesla is just as obsessed, in his own way, as Angier and Borden. And he’s involved in his own mad game of one-upsmanship with Edison.
That’s not really the message, though. It’s not that Borden’s twin “couldn’t” love Sarah - it’s that he simply did not. He may be a twin, and he may share his brother’s obsessions, but he’s ultimately a separate person who falls in love with another woman. Neither Borden is capable of faking love with the woman he ISN’T in love with.
Angier doesn’t really have “clones” per se. As the hat scene indicates, ALL of the Angiers are the “original” Angier. This isn’t Star Trek style transporter wackiness, where the signal buffer degrades or whatever. This is Magic.
Indeed he was!