What's wrong with executing retribution on the thief for his stealing your kill? He steals your money, you steal his life. Seems fair
That is only a valid means of thinking IF you're not intending to play as a group to take down bigger targets.
I think what might be better is if you engage a Bounty / Warzone ship; that you receive say 5% of their value for participating, this doesn't stop the whole 'Hey you stole my kill you b*stard!' aspect, and likely won't cover ammo costs in most cases. Still it would provide the idea that you are still provided with Credit for the Kill (for Missions) and a /token/ Bounty for participating.
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When it comes to Wing (when Wingmen update is release), the idea of Sharing the Bounty based upon participation then starts to make more sense; where it could be an option for Kill Right (Final Blow), Equal Share (Shared Equally) and Pack Mentality (Shared based on Damage Dealt) with it being the Pack Leaders decision, which to set.
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Again though every participated kill should use Pack Mentality for the +amount to your Elite Rating or if >25% Kill Credit for Missions; as one of the most frustrating aspect isn't just losing out on the actual Bounty Credit but Kill Credit as well. As it discourages Teamwork; I'm not suggesting that Teamwork should have major encouragement, as with this concept it will pays to fight in smaller groups of individuals but it does suck especially in Warzones fighting in groups of say 3-4 Eagles taking down bigger ships together that only one person will ever get any of the Credit.
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There was already the update to help alleviate the NPC Kill Sniping that was occurring, which still does happen (they should never qualify for the Credit, sorry but they shouldn't)