I think a lot of it has to do with the kind of villains that Diana faces vs what Batman faces. The Supernatural is a lot more prevelent in the corner of the universe Diana inhabits. Batman should have more restraint because he's dealing with mortals 85 percent of the time. Diana on the other hand is dealing with gods, monsters and other entities that exist on a far more dangerous scale and power level. For a large percentage of Diana's rogues even death might not be a permanent solution. Ares and Circe are gods and thus probably have access to several forms of resurrection. Cheetah like wise is powered by supernatural forces and thus can be brought back even if killed. Cyber, Poison and many of the other human villains are legacy villains and thus have characters waiting in the wings to take up the mantle or ways of surviving the death of their physical form in the case of Dr. Cyber. It's interesting out all the main DCU characters Diana is probably the one with the most lenient policy when it comes to killing, but she arguably has the one rogues gallery we're killing would be the least effective.