The effect of this card is optional; you do not have to return it to your hand.
The effect of this card activates no matter how it is sent to the Graveyard. It can be destroyed, Tributed, discarded from the hand, or sent straight from the Deck to the Graveyard.
If your "Sinister Serpent" is on the field and is destroyed during your Standby Phase, you may return it to your hand during that same Standby Phase.
If "Sinister Serpent" is destroyed as a result of battle with "Dark Ruler Ha Des" or "Dark Balter the Terrible" you cannot activate its effect and return it to your hand during your next Standby Phase, or any subsequent Standby Phase.
You cannot activate the effect of "Sinister Serpent" if "The End of Anubis" is on the field.
"Necrovalley" does not negate the effect of "Sinister Serpent".