I'm looking for a trope where there is a group of people and someone kills one person and blood/guts go everywhere and it splatters all over the audience.
There are several tropes dealing with gore and blood splatter. Ludicrous Gibs for example is a trope for when there is simply an excess amount of gore (think Evil Dead). But I think closer to what you are referring to would be something more like Gorey Discretion Shot combined with Blood Spattered Innocents
I think just about every episode of Supernatural starts this way.