Aprophis wrote:

ServantofJustice wrote: How exactly and why does this happen? We know not all from murder/suicide and etc become that but Sluaghs are humans who for some reasons becoming involved in those tragedies become a BA spirit?

Rusalkas are also former human spirits that became BA for certain reasons.

If you read the Sluagh's description, you'll notice:

They are not recommended for keeping because they are annoying and difficult to get rid of

And that's why CH classified them as BA. In reality they're probably DA.

Same goes for Rusalka:

They are not recommended for keeping because they cannot be controlled and have an indifference to human life.

But to get at the core of your question. BA energy has a kind of corrosive/contagious longterm effect so if someone is affected by it long enough, they can get infected/infused with it.

ServantofJustice wrote:I forgot to mention; there is also one more type of human BA or former human called a Dybbuk?

They're DA too or BA? It says they violate free will to complete something they didn't manage to finish then move on after.

It is not recommended for keeping because they are difficult to live with and in some rare cases never leave the human.