Oh, you're absolutely right, in the same way I'm right in saying The Best of Both Worlds is Patrick Stewart getting alien-abducted by bondage zombies and then rescued by the power of friendship.
Bring enough contempt to the table, and anything at all sounds stupid. But the problem there isn't with the thing.
I dunno, I thought it did a pretty good job explaining itself, not least in the closing scene where, as a side effect of the same sterling character work it features throughout, it literally explains itself. What did you find stupid about it?
The issue for me wasn't that it was under-explained or anything.
My issue is how jarringly out-of-place it feels. In the midst of a futuristic space opera, they shoehorn in a notorious serial killer from the Victorian era. JMS meant it to be cool, but instead I found it laughable. B5 wasn't Doctor Who, where mashups like that happen once a month.
Why does Jack, of all people, attract the Vorlons' attention? Why do they choose him as their tool? Why didn't the Vorlons interrogate Delenn themselves, or make a more recent interrogator? We're told he's uniquely-suited to his task, but nothing about his character convinced me that's true.
Why even were the Vorlons in doubt about Delenn and Sheridan? (I don't remember if there was anything leading up to the episode, but I admit it's been long time, so maybe there was something. I just vaguely recall it felt a bit out of the blue, but I could be wrong there.)
Regardless, there's really nothing about Sebastian or the episode that was improved by making him Jack the Ripper. It's my personal worst B5 episode.
Well, that's certainly fair. I don't share the same view, but that's hardly a surprise; whatever else there is to call this episode, it's certainly high-concept, and those do tend to be the more divisive ones among the fanbase.
For me there's something akin to a "twist budget," maybe; a show that does it too often or by too much of a leap starts to look overly pleased with its own cleverness. This doesn't reach that threshold with me, and indeed I thought the twist here actually did a lot to add depth and pathos to the character of Sebastian. For him merely to have been a sneering villain would probably have worked, set against Furlan's and Boxleitner's performances, and given after all this was a story about Delenn and Sheridan. Making him a repentant monster, with a unique perspective on their values and choices drawn from the horrific miscarriage of his own, turned what could have been just a sneering villain into something almost as pitiable as frightening, and shed new light not only on Alexander's performance but, in a narrative sense, on the depth and extent of the Vorlons' involvement in human history - a topic on which the story would later have much more to say, and putting in some work to help set that up here would help those later occasions likewise avoid tipping over into the "too clever by half" sort of twist.
That said, the specific questions you ask here have been asked before, and JMS has answered them. Those answers are collected on the Lurker's Guide page for this episode [1], and I think are worth your review; even if you find they fail to persuade, they'll at least still yield insight into why the story was written as it was.
Bring enough contempt to the table, and anything at all sounds stupid. But the problem there isn't with the thing.