Hinduism, Initial concept of Shiva, Trinity...

[NOV]

another school of thought believes that originally the gods worshipped by tamils were thirumal and amman, other than the different village gods.
all other present day gods were "imported."
both these gods were dark-skinned, as opposed to all others.
later the thirumaal worshipped by tamils was absorbed to the "imported" vishnu.
it also explains why siva has no generic tamil name.

on brahma-vishnu-siva trilogy, brahma is not permanent and is created every yuga. you would have seen brahma coming out of the navel of vishnu.
on why must there be a god of destruction, everything created will eventually be destroyed and birth-life-death is a cycle.
destroying is not bad - destroyer of ego, ill-thoughts, anger, greed, etc
brahma - Generator
vishnu - Operator
siva - Destroyer
=
God

many of us know the link between sivalingam and the phallic symbol - there is nothing to be ashamed of, as fertility is the root of existence.

as I said, all these are schools of thought and there is no way anyone can prove which one is right or wrong.