If a murderer gets killed, then in the gandhabba state he/she will be likely to cultivate similar bad thoughts (vaci sankhara) that were regularly generated during the life as a murderer.
Then his/her fine gandhabba body can undergo a transition to a hell-being gradually. Then his/her “bhava” can change to the new bhava of a “hell-being” and at one point, will make the cuti-patisandhi transition to a hell-being. This is an important point.
When inside a solid physical body, gandhabba cannot transform significantly, because it is trapped inside. But once of out of the body, gandhabba can transform relatively fast. This works both ways, to get to a “bad bhava” or a “good bhava“.
Waharaka Thero had some abhinna powers, and he was able to see some people’s gandhabbas transform in real time to animal gandhabbas. He described that one time he saw a human gandhabba transform over time to a bird. It started at the head and the head gradually looked like that of a bird. Then transformation spread to lower parts of the body ending up with the transformation of human legs.
– Even some normal people can see gandhabba bodies (due to punna iddhi), and this could be the origin of picture of human bodies with various animal heads that have appeared in various cultures; see, “Abnormal Births Due to Gandhabba Transformations“.
This is why one who has cultivated jhana (Ariya or anariya), WILL BE born in the corresponding brahma realm, even if there was kammic energy for the human bhava left at death.
In the same way, one who has attained Arahantship, the gandhabba will altogether disappear from this world of 31 realms and attain Parinibbana (even if there is more kammic energy for the human bhava left), since the it cannot transform to any bhava.
Those last two transformations happen very fast, instantaneously; there is no intermediate gandhabba state. In the case of a anantariya papa kamma (like killing an Arahant or killing parents), the transition to a hell-being will happen instantaneously. That is why they are called “anantariya” (na + an + antara or “no in between state”).