I think she'd probably be able to handle it, as long as she gets the opportunity to commend the deceased to the afterlife afterwards. Ellen's made a point of doing some form of final-rites prayer for organic sentients that she's killed, on the grounds that she's pretty sure she'll wind up dead alone in the wastes one day and hopefully some stranger will offer the last rites for her when that time comes. The draining-lifeforce aspect is... well, she's worked with a whole community of people who used to be outright cannibals and who're currently living the lives of vampires in a desperate effort to keep from reverting to an overwhelming psychological compulsion to eat human flesh. Like, I'm pretty sure the Family is walking around with straws jammed into donor blood bags like really sticky CapriSun packages. And Fawkes, before he put his head back together after his FEV infection, is almost sure that he outright ate people, although he honestly doesn't remember much from that time. Ellen has probably dealt with worse than a mere lifeforce-drain kill or twelve.