because its seen as killing life i think, not sure though

Because pleasure is bad.
Worship the almighty being or you shall suffer eternally!

A "perfect being" apparently decided that he's give us the ability to give ourselves pleasure, but fucking hell if you use that ability you're going to hell.

"it is better for your seed to be placed in the belly of a whore, that to fall upon the ground..."
It goes something like that, but i'm not sure it is actually in the bible or not.

I just googled that, and I'm finding multiple sources saying that that phrase is not found in the Bible. However there is the story of Onan who fucked his brother's wife after his brother died and he pulled out before cumming, thus "spilling his seed" onto the ground. But that was sinful for different reasons, Also it's not really masturbation since it was with another person.

Fear.

The entire concept of religion is based on fear.
When it was initially created, religion was just a way to instil fear in people to make them obey them (as the church does).
The church does whatever the fuck it wants, says whatever the fuck it wants, and just says that this is the word of God and must be stuck to. Hence why churches are exempt from tax. They use religion for power.

That has jack shit to do with masturbation, you're probably just mad you mad to go to church today. Masturbation is a solo thing (we're not talking mutual masturbation). It's all about you and your pleasure. Sex is intimate, masturbation is not.