1 kill = 1 creature.
Doubler "At skill level 50+, 2x Stamina usage in combat in return for 2x gold/xp...." and these global quests aren't for xp/gold gain.
Kill streak doesn't increase with doubler so why should count in the Global quest?
I agree with this. If we start allowing doubler to count as extra kills then it should also affect your kill streak. To make it count as an extra kill is a bastardization of what the skill really is. It's getting more XP/Gold for using additional stam not getting extra kills.
I understand the ideas being put forth regarding making the load on the servers lighter, faster kill totals, greater rewards, etc. but in my opinion it is not a good idea.
I would suggest instead a special potion or weapon that must be worn in order to get the double kills. Something like the potion that allowed you to see the creatures in the new epic quests or the runes need to kill the elemental titans - I know that those a sketchy ideas and likely tough to implement - I just wanted to give some alternative because, I do not like the idea of changing a game buff to do something it is not supposed to do in order to make everyone happy for an event that occurs every couple of months or so.