There's no accounting for what might happen when emotions run high within drug/alcohol/party circles.
The nicest people have been seen doing craziest things.
Was this the case here, or was it a brazen, sadistic, premeditated murder?
(confession letters notwithstanding)
In reading the note, Robert claims to have hit Angel in the head with a hammer 3 times. The first two were not effective in stopping the assault. This suggests a level of hesitance on the part of Robert. The number of hits would be verifiable if the body (head specifically) was recovered. If I were to hit someone in the head with a hammer with the intent to kill, I believe one blow would be all that was needed. (Maybe it was a rubber mallet? A "picture hanger" hammer?)
The cover up seems the worse crime. These guys could have gone with self defense in court, but once you dismember and dispose of a corpse... not so much.