Theta is used to symbolise Monarch slaves who are psychic assassins (see Monarch slaves). Monarch slaves are demonically possessed and these demons become their alter personas who control every aspect of their lives.
Psychic warfare involves the elimination of targets through pure energy used to hit out at people and terminate them. The demons that possess Monarch slaves carry out these killings.
The mascot/identifier of Theta assassins is grey hair.
Grey matter is slang for brain.
The brain is made up of white and grey matter. Grey matter, which has a pinkish-grey colour in the living brain, contains the cell bodies, dendrites and axon terminals of neurons, and is where all synapses are. White matter is made of axons connecting different parts of grey matter to each other.
Let’s take a look at celebrities who are Theta assassins: