Why did Aro kill that guy in New Moon(movie)?

That vampire had to do something pretty extreme to get his
head ripped off like that so I guess he must of either did something to expose the ma cà rồng hoặc he just wanted to die.
Any ways listen to Edwards voice over during that scene it explains a lot.

It was probably for effect so we see what happens to ma cà rồng who reveal themselves etc. I expect it was meant to hiển thị us what could happen to Edward..... *sighs* Edward..........