User blog comment:FairyRaven/Fairy Tail Chapters 338, 339, 340: Review and Commentary/@comment-86.66.207.63-20130706094810/@comment-7418318-20130706112235

Someone other than Angel killing Yukino and losing the keys to Lucy wouldn't really change anything. It's still the very same thing that happened with Karen and her keys, only with another "intermediate".

If she's killed I think it'd be better if her last command to her spirits was to seek out Lucy and bring her the keys (with Virgo and Angels spirits approaching Lucy on their own, that seems possible) or if Lucy and Yukino fought side-by-side when she's killed.

Or preferably Mashima could come up with a non-lethal reason why Yukino turns her keys over to Lucy for real. She could suffer similar symptoms as Lucy's mother and have to give up being a mage, or she might be hurt and left unable to fight the coming darkness or she might marry and give up being a mage in favour of founding a family.