Thanks for asking! I have in fact answered the question on worgen instincts before, as well as some others on worgen deathknights you might find useful. Your question here about worgen deathknights and instincts was asnwered as well in CDev Answers Round three, and the quote below claims the instincts were shattered and remain so.
After being raised by the Lich King and regardless of their past life, almost all death knights embraced the honor of their new status. However, there were the rare few that were reanimated and retained emotion and memory, and in some cases, fear. These “unworthy initiates” to the dark brotherhood were then disposed of, fed “alive” to ravenous ghouls. Death knights afflicted by the worgen curse had their feral instincts shattered in favor of the Lich King’s will. This held true even after their freedom from the Lich King. – CDev Answers Round 3
- How are worgen death knights possible? link
- Do worgen death knights still have instincts? link
- Why are worgen immune to conventional undeath? link
- Are worgen still immune to undeath? link
As for your post, let me see if there is anything else I can add…
Can worgen Deathknights shapeshift?
Reasons why not
Normally, I would argue that worgen deathknights cannot use human form. When worgen die, the default form they return to in the worgen body, seen in Wolfheart and the comics. Shapeshifting is a gradual physical process and a magical ability, likely drawing on the power of Elune and Goldrinn present in the curse itself. However, while we have seen both kaldorei and humans able to become worgen, we have only ever seen humans capable of shapeshifting… which has interesting implications. If death cuts you off from Goldrinn and Elune, leaving you with the physical worgen body that is left behind in death, a reanimated worgen corpse would - in theory - not be able to shapeshift.
Reason why
Human form is only possible to worgen that are clear minded, such as cultists in the Grizzly Hills and Gilneas, and Genn Greymane. Some worgen have to take the Ritual of Balance to help them move on from traumatic memories, such as Halford Ramsey, the player character, and the human king Varian Wrynn. Since it is emotions and instincts that prevent one from being able to shapeshift, and almost all deathknights have little to no emotions, there would be little to hinder them from shapeshifting.
In any case, out of all the worgen deathknights we have seen that have never shapeshifted, we do have one that is said to be a deathknight worgen, and is seen in both forms.
https://wow.gamepedia.com/Grant_Lazarby
Could bestial instincts return?
The CDev response claims what worgen deathknights have their bestial instincts shattered and replaced by the Lich King’s Will, and that this stays so afterwards. However, while community developer responses are a bit better than twitter replies, they still are not much to go on, and some use faulty logic or become outdated as the game adds more lore.
We have seen where deathknights have been specially selected and trained to be emotionless soldiers for the Scourge army, devoid of remorse, regret, compasion, and even memories of their past lives. However, those emotions can be felt again for a fleeting moments at a time, and some deathknights have managed to regain some scraps of memories of their past lives here and there, such as Thassarian. We also have some deathknights who still eat food, drink alcohol, sleep, and smile, but only as habits left over from life.
Therefore, theoretically, if your worgen was a worgen for an extended period of time before being turned into a deathknight, they may have both memories and habits left over from being a living worgen, and might have fleeting moments of anger, the need to hunt, instincts of territory or guardianship, or other bestial tendencies. But how much of it is actual emotion and instinct, or mere habit or an act to make others more comfortable, that would be difficult to tell.
Bacons to you anyways! Generally as a deathknight, you get to throw out everything you know about worgen, and rely only on what you need to know about deathknights. Which is great, actually, because being forced to research both would be a lot to ask of a player. However, if you are a deathknight and do it well, and would like to incorperate worgen elements, you can use the “fleeting memories”, “force of habit”, and “the act” reasons to add those bestial elements to their behavior.