So I saw a video on youtube showing NPC reactions to dead body. First I thought it was a part of quest and that it doesnt happen usually so I decided to try it out.
So I killed a random guy in Whiterun, as usual few NPC came to body and were like "Oh..What Happend" BUT, after some time of Waiting near body a group of NPCs surrounded body (while keeping distance from it ) and commented like "What happend here"," Is she/he Dead" etc. Then the guard came and ordered NPCs to get back saying : "Alright now .Everyone back away.Let the law handle this" After that he interrogates you depending on what have you been doing at the moment

Here is a link to that video http://www.youtube.com/watch?v=LST3TqZqfPc
So my questions are : Is this suposed to happend always and has anyone else experienced this ?

I'm pretty sure that has happened to me. I don't think it happens all the the time but I suspect if you stay near the body for a long period with your weapon drawn (as the guard mentions in the video) it activates that dialogue.

Pretty neat find, I've never had that specific dialog but I cannot stand next to a dead body with a weapon drawn for too long before I have an archer filling me with random arrows

The one I love is using a shout the guard comes over and says
Guard -stop using those shouts there making people nervous
Me -I can do whatever I like
Guard- ok there is no law against it but stop anyway
Me- whatever
Guard- iv got my eye on you