death to capslock

That might simply be a matter of skill: if Frank was less effective than Moody -- and Moody is generally portrayed as having been highly effective -- he might have had to resort to killing more often, regardless of any preference for bringing them in alive.

(Although then again, given that convicted Death Eaters would most likely be sent to go mad in Azkaban, maybe Frank just reckoned that killing them in combat was the merciful course of action. A clean, quick, honourable death, and all that.)