Interesting and knowledgeable discussion.
We all have gone through certain 'rights of passage' in our youth. Only experience and age can assist us with making seriously better choices particularly after a night out on the town where our brains are pissed.
{ie: I now take a taxi as opposed to driving after getting impaired.}
Possibly if one is not up on the differing life-style sexual choices (TS, TG , laby-boy or khatoey etc) and such then quite possibly the young Marine fell hard for the murdered party at the disco.
Possibly had his 'USMC' soldiers ethic pride removed upon discovering something he may possibly have never wanted to bargain for prior to going out that night.
I ask myself if his buddies realized who and what he was attracted to before the couple left the disco.
In finality he made a very poor choice. Killing a civilian is never justified.
A soldier is instructed to show passive restraint if not threatened with harm.
If he can conclusively convince others beyond a shadow of doubt that his life was duly threatened he has a miniscule chance for his rage.
The fact that he may not have reported the 'incident' in a timely fashion will definitely not work in his favour.
The USMC knows what they must do in the case of manslaughter and will not allow this young man to plead temporary insanity (IMO).
Caveat: I am a 'newbie' on this forum. Thank you for allowing me the initial courtesy of this post.