One could say that this question...
...is just an introduction. Something you should clarify before choosing what you want. Who you are... it defines origin, identity, reference points in your world. It defines biases, prejudices and limits. It defines where you stand, your location.
Without moderate understanding of your location (and the location of others) any direction would be meaningless. You cannot choose where you wish go, unless you know your location. Being unaware of your biases, unaware of your vantage point, you might choose your direction badly.
Vorlons spent the whole time considering who they were. Due to this, Vorlon society rarely got around to reforming anything. They valued tradition and liked keeping traditions, even if such traditions were wrong. They had forgotten that location is not everything. Without direction, location means standing still.
Shadows preferred choosing direction with little concern for location, and such directions were deeply flawed. Sometimes, their activity changed things for better, but equally often it just destroyed. They had forgotten that without knowing your location, any direction you choose is random.
Explaining that to Sebastian would have dubious value. His biases clearly ran deep. Forgetting change and only dealing with identity, he probably no longer correctly perceived who he was. Such a person cannot change quickly or easily.
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Delenn had plentiful reasons to refuse assistance from Vorlons, if such were their methods, and such was the price. I think I recall even Sebastian telling that she could refuse, but that would mean admitting unworthyness for the cause.
Then again, one should remember that her cause was not the Vorlon cause. How would refusing a cruel test for the Vorlon cause show her unworthiness? Clearly this is not possible, and in my opinion she should have refused.
Why she chose to undergo such torture... is not clear. Probably because she deemed assistance necessary, worth more than any personal cost. Perhaps she thought that if she were to die, someone else would try and succeed.
That approach is however justified only to certain extent. You cannot value the lives of others unless you find some reason to live your own life. What she also forgot was that assistance with corrupted purpose... cannot really assist.
Given that Vorlons were willing to use such methods, their long-term assistance was worthless. They could only balance out Shadows, which meant both sides inflicting equal damage. The benefit of bending to Vorlon wishes was clearly illustrated by Delenn's later conversation with Ulkesh.
"The stories we've heard about what happened at Z'ha'dum. Are they true?"
"Yes."
"What do you intend to do about them?"
"Nothing."
"But if Sheridan is still alive, if there's even a chance."
"Irrelevant."
"Irrelevant? How can you say that? You know how much depends on him."
"He has opened an unexpected door. We do now what must be done now. His purpose has been fullfilled."
"You have the power to send a force to investigate. If he's alive, he can be rescued."
"No one returns from Z'ha'dum."
"That's what we thought about Anna Sheridan. You were wrong then, you could be wrong now. Are you afraid to admit that you are capable of another mistake? He pledged for you, worked for you, for me, for everyone here. If you turn away now, if you abandon him to die on Z'ha'dum, I will have no more respect left for you. Do you understand me?"
"Respect is irrelevant."