What if my pleasure is dependent on someone else's pain?
Then you shouldn't pursue it.
By what standard of morality should I refrain from causing harm if it gratifies me?
Ethical Hedonism has roots in Utilitarianism. ie, people should act in a way that creates the largest amount of net pleasure for society as a whole (pleasure minus pain).
He defines hedonism "as an introspective attitude to life based on taking pleasure yourself and pleasuring others, without harming yourself or anyone else."[27] Onfray's philosophical project is to define an ethical hedonism, a joyous utilitarianism, and a generalized aesthetic of sensual materialism that explores how to use the brain's and the body's capacities to their fullest extent -- while restoring philosophy to a useful role in art, politics, and everyday life and decisions. https://en.wikipedia.org/wiki/Hedonism#Michel_Onfray
But if my pleasure involves the torture, rape, and murder of as many people as possible then why should I care about the well being of a society that's going to eventually be wiped out anyway? Obviously I don't take pleasure in such things, but if you get enough people who do then "ethical hedonism" doesn't really have a leg to stand on as a valid philosophical viewpoint.
I think it still is a valid viewpoint. Society should seek to maximize net happiness, and all of the acts you mentioned create far more misery for the victim than pleasure for the perpetrator.
That's a pretty unsubstantiated claim. You'd have to be able to accurately gauge the feelings of euphoria and psychological turmoil in each subject before reaching that conclusion.
And by what standard should society seek to maximize net happiness? People have varying ideas as to what brings about happiness, which in itself cause societal conflict.