the only first peroson assassinations I know in a shooter are from BF3, and that perfrms well.Won't break your immersion,and fast too.I say add 1st person if there are 1st person vehi views, that way, you're a;ways in first -person no matter what you do.
"Yeah, but 100% first person isn't what Halo is about. Ever since the first Halo, Vehicles mounted guns, and turrets have been in third person.
I personally believe it should stay like that.
For thought who want a quick kill so they can quickly move to their next target can still can use the first person beat down...but for those who want to rub a kill in someones face they can assassinate the guy and make them watch themselves die in third person.