Ok, I have to share this since its the first time this has ever happend to me. I just completed the Companions Main Quest line and now am Harbinger. I ask Vilkas to be my follower. I go to many cities buying up Igiots and other stuff to build weapons and Armor. After all that I headed to my Heathfire Home with Vilkas in tow. As Harbinger I want to start upgrading all my Companion weapons. So I take Vilkas Two hand Skyforge steel sword and make him a Nordic two hand sword, that I think would look bad *** on him and it does. Once done and after off loading other gear, we head to Whiterun. On are way, we run into three Imperial Soldiers . I set up and attack them easly dispatching them one by one, yet as I'm finishing up the last guy. I noticed that my health is very low and should not be that low. Next thing I know, I find Vilkas swinging his brand new two hand sword at me. As I'm dieing, I'm thinking to my self he must have really wanted to be the Harbinger. Just waited for the right time to kill me and say I died in battle...

You probably didn't notice that you're hitting him and he went hostile. Happens quite often to me in the heat of the battle.

There can be only one!

I don't think Vilkas condones killing soldiers on the road. He's kinda a goodie two shoes.

By the time I get to higher levels, the only NPCs that I would be afraid to confront are my followers. I give them top-of-line gear, so if they do come after me, it is going to be a serious problem. I had Rayya kill me once. I was practicing Soul Trap on my horse at Lakeview Manor and Rayya was patrolling the grounds and she kept saying "don't do that" over and over until finally she had had enough, pulled out her scimitars (legendary+++++, enchanted to the max) and killed me in two swipes.

If thats the case then I wish I had the option to kick and recuit new Companions. I dont need any goodie two shoes in the companion ranks..

No one does, but once they become the companion of the esteemed dragonborn anything goes, from shoplifting I<3Thalmor mugs&plushies to killing men, stripping them naked, reviving them and making them run all over the city.

That annoyed the hell out of me when you accidentally hit a follower, and they attack you... this is why I got a mod for it.