User blog comment:Bunnyllove/Have Natsu and Zeref meet before?/@comment-24066070-20131111101402/@comment-24066070-20131111121725

Rufus can "memorize" and "create" magic just by witnessing it, he doesn't need to get hit by it and he doesn't need to launch it from his own limbs. About Zeref's immunity against his own magic, if the death aura can't do the trick then nothing will.

Rufus killing Zeref would be far more creative and unexpected than Natsu using Nakama Plot Lightning Fire Dragon God Secret Slayer Technique.