Thirsty Mfs (half rant)

That sounds frustrating but if it's a one off incident, I'd just move on.

Bad play could be a reason.  Another reason is people either focused on K/D ratios as if it were the point of the game, Kills so they don't lose rank points, or kills to accomplish challenges.  All those three things can lead people to do stupid assassinations selfishly, than just realize to play with their squaddies will lead to more net kills.  You know how it is, many people aren't team oriented, and there are things in this game which weirdly encourage you to not play as a team.

I know that assassinations aren't always wrong, when sometimes they might seem like they do.  In the super early game, right after drop, you might kill someone if you don't have a suit of armor and haven't picked up enough ammo and a fight is on.  Because you're pretty much useless anyway, and if someone bothers to stop your attempt, maybe your squad mate can kill them, or he'll pick up your box later anyway if you live.  I've seen it in late game because you just never got enough shields or health, and/or your armor is wasted and you do have the cover so you're trying to make a box to loot asap to bring yourself back for the final fight.

But these are rare.  More often than not, yeah, people are trying to register a kill and not thinking straight.  *shrug* that's public play, you know?