Stick with me here. I don’t really think the knight is a person. I thinks it’s some sort of mutated titan spawn or like a fragment of a titan. No Lightner or Darkner can change shape like they can and they are so much stronger than any other enemy in deltarune being able to fully wipe out an entire party member in one to three hits if they don’t have special armor. No one comes close. And they like drool and roar they never speak so. Plus I feel like narratively it would, at least for me, be pretty weak having the knight as an actual person. 
“Erm actually a mutated titan spawn wouldn’t have a motive” OH MY GOD SHUT UP NERD. But to answer the question it would actually as Titan spawn exist to attack and destroy and the knight is doing that by starting the roaring. This is what I’m going with till I’m proven otherwise.
Not to mention the design similarities of both Titan spawn and the roaring knight just being black with white outlines
One small issue.
The Jockington scene.
What… jockington… scene
The Jockington scene.
MY GUY TITANS ARE DARKNERS
THEROOOOY JEEENKIIINSSSSS
sorry I have to do that whenever anybody misspells theory as theroy
doshdangit pixl beat me to it
ANYWAY
the theory
first off, this theory has been made an absurd amount of times before, and there's just so many counterarguments, and almost everything of "certainty" you say here can be disproven pretty quickly
in Undertale, when you get to LV20, you can DESTROY THE WORLD IN ONE SWIPE. we've never seen what happens when a Lightner reaches LV20 in the Dark World, so it could be that high level Lightners just get crazy powers like that. that, and, it's more than possible that it's the armor that grants the Knight its abilities.
tbh I have no explanation for this one
that, plus the fact that the Titan[1] doesn't actually roar. they make various noises, but no roars to be found.
that would give away their disguise
why would they want to do that
all we have on the Titan's[2] destructive tendencies is the Prophecy[3] which has already been subverted time and time again.
oh no am I becoming Volnar
Well that’s just how I feel. I wouldn’t mind if it was a charecter not commonly speculated as the night like tEMmie but Dess ro carol would SUCK
it depends; have you developed an excruciatingly critical view of all theories involving motifs whatsoever?
No not really it would just feel worth to me honestly
I’m not overly critical of theory’s or anything just I is t think Toby would have any obvious connections like that. He is tricky Toby after all
I was replying to Tim actually
Are they though? I always thought they're a third different type so they can exist in both dark and light worlds
The Knight is probably that as well, that's why it can retain its form in the light world or something
You either die a hero, or live long enough to become volnar
This does sound possible, but... ALWAYS BET ON VESSEL KNIGHT
I am a Wise Words in Flower Man believer if that answers your question
Cahem...
Lancer
Rouxls
Starwalker can get minuscule to fit Kris's pocket
Tenna's bare existance
Spamton can Glitch his face as he is a digital concept
Jevil
Netskie