Right, so..I was heading to Jorrvaskr to pick up a quest or two, and I found Athis ( http://elderscrolls.wikia.com/wiki/Athis ) laying dead on some rocks behind the building, near the "dining" area there. I tried loading several older saves, even some from when my character was only level 10 or so (It's 30 atm, and level 10 was around 20-30 gameplay hours ago), and even in those old saves he is still there, dead. I have no idea when or why this happend.. I remember seeing him when I first entered Jorrvaskr, and he was alive then.

Is this supposed to happen or is it a strange bug? If I resurrect him with the console will everything be alright or will it "mess up" my save in -any- way?

Do you have dawnguard installed? any vampire attacks in Whiterun? I don't believe there is any scripted event that kills him. Got no idea.

I rather liked him.

Nope, don't have Dawnguard, and I use very few mods.

i read someone said this happened to Ria on theirs,he may re-spawn later i hear that happens to some companions,but idk,after i got hearthfire it brought skor back to life,lol

I've come across the companions out in the wild on their own. Came across Ria and Aela fighting a mammoth. Probably a good thing I was there cause that thing just about killed them both.

So... maybe in your travels you were in the area Athis was, maybe he was under attack and somehow died. The game then re-spawned him in Whiterun but dead? I've heard of people taking companions as followers, they got killed and then their bodies respawned in Whiterun. I think one may have been Athis and he was missing his head...