When a person goes of on a tangent and completely destroys the conversation or atmosphere.
You are having a nice evening down the pub and you are explaining to the assembled crowd of friends something exciting or retelling an old story and you wander off on a massive tangent which at the end has either offended, confused, alienated or stunned your friends into a silence as they contemplate your mental health. Or one of your friends does that.
That is Tangentacide. Often followed by people sneaking away from the table, needing the toilet or finding out their friends are in dire trouble and they must leave.