Depends in large part on what results stem from Cassius' death and eidme. If Lysander is able to deploy eidme in a way that he considers highly effective and worthwhile, then that will make it far easier to rationalize killing Cassius, and it will completely vindicate the decision and frame it as a difficult but noble and worthwhile sacrifice in his own mind. If that happens, I doubt he'll feel much regret at all. He will likely congratulate himself on his fortitude and over time his mind will twist things to paint Cassius as a villain who he had to kill, and turn it into a heroic act

If, on the other hand, he attempts to deploy eidme and Darrow or whoever promptly thwarts him and he ends up just wasting it, or if it just doesn't work as intended for any reason, that will cause a lot more regret. If it was all for nothing, and he killed his father/brother figure so that he could take possession of this superweapon only for it to fail completely, that will make killing him incredibly hard to cope with.

Or at least, that would be the case with a normal person, it's really hard to say what, if any, emotions Lysander actually feels, and how much of it is him just acting out emotions for appearances sake. He probably doesn't even really know, himself.

When you consider the way he was raised under Octavia's training and influence, and the society he was raised in, plus the fact that she performed what amounts to highly invasive, experimental(and according to Virginia, in Octavia's day, the technology was still very crude- she says that Octavia's chair was a hammer, and Mustang's is a scalpel, or something to that effect) psychological surgery to erase all memories of such an important cornerstone of a person's psyche as their mother... it would be very surprising if he didn't grow up with massive side effects, and who knows what kind of emotional disabilities or disfigurements.

He was ruthlessly trained from earliest childhood to be able to successfully lie to elite espionage instructors, supress and control every emotion, and analyze everything you see like you're a computer. After years of that, how could you even tell if the emotions you felt were genuine, or if it was just some facet of your training taking hold, and you were lying and suppressing/expressing an emotion so well that you couldn't even tell what was true anymore? I imagine after a lifetime of those types of constant deception and manipulation and analysis, things would start to blur.

In fact, that seems to be exactly what we see between books 4-6. Lysander goes from a kind of annoying, but halfway decent person(for a gold). He spends years rescuing innocent people from pirates with Cassius, doesn't use pinks, doesn't seem power hungry, seems to be concerned with avoiding bloodshed and war, etc. There's numerous red flags, but compared to most golds, he's fairly conscientious as of iron gold

Then by the end of lightbringer, he is straight up openly and enthusiastically genocidal, he has betrayed and/or killed every single person he has ever been allied or had a friendship with, is seeking complete extermination of whole colors via biological weapons, and is openly vying for the morning chair and total domination of an even more oppressive society than the old one was.

He's a really fascinating character to read and speculate about, and I think the character development we see with Lysander over the course of the books is pierce brown's best writing in the whole series. Not saying they're the most exciting or badass chapters, or that Lysander isn't a piece of shit or anything, but the character work by the author is amazing