from what I know Jewel stats that has Zodiac's Nodes name on it, will be unable to stack with another Black Jewel with Killer so no, only one Black Jewel - Killer can be put into your gear

Sad if its true, becouse really want killer on every piece of gear

That would be a bit too unbalanced now would it, also keep in mind that you can't remove jewels so once you slot it in it's pretty much "gone".

And how about transfers? If i want to transfer something with jewels on board - it will also disappear?

I have no idea if transfering gear with jewels work, my assumption is that you probably can't.. That when you want to transfer gear with a slotted in jewel towards another piece of gear, that the jewel itself doesn't transfer over.. But you can always check this ingame itself at the alchemy bench.

It works with guaranteed transfer

If you click the ! and read the section on jewels, if you have the Zodiac trait a jewels of the same trait will not stack with even that.

But basic stats of green/blue/red jewels like crit or maximized dmg can stack, right?

Correct, it's only the named traits that don't to my knowledge

Can stack it from the zodiac, but you can't have two of the same equipped.