Yet—and it’s obvious where I’m going with this—“Spock murdered EvilKirk” is not a meme. ...[W]hy does he get a pass while Janeway is condemned?

I'm gonna cheat here a little bit. There are lots of things about the Tuvix debate that really are not about Tuvix; we've seen Tuvix elicit underlying opinions about everything from abortion to the trolley problem, and we've often seen thinly-veiled misogyny lurking beneath the surface as certain folks appear interested in finding any flaws in Captain Janeway they can.

So, yes, somewhat trivially, I think that Spock does not get condemned in part because he enjoys the luxury of not being targeted by misogynists. I think it's a small part, but surely a part.

I think the reason there aren't memes is because, well... "The Enemy Within" is an old episode, it never attracted attention through debate, and also it's a gross episode where EvilKirk straight up attempts to rape Yeoman Rand, and then Spock makes a joke (! Spock! A joke!) about it at the end of the episode. I'm sure it's an episode numerous folks have tried to forget. (I know I have.)

So, in terms of using the episode as a barometer to evaluate our own reactions to the two situations, I think there are overriding contextual factors that drown out any insight we might gain.

As an aside, I also think there is a pretty straightforward argument that EvilKirk (and GoodKirk, for that matter) was not mentally competent. He was, by definition, the remnants of an individual who had had a significant piece of their person torn away from them traumatically.

I think there's actually an odd but useful comparison to dementia here: dementia does cause some individuals to behave "out-of-character", immorally, or just meanly. If there were a "transporter reintegration" equivalent to treating dementia, and the patient said, "No, I want to stay like I am"... then I think probably the patient's wishes would be ignored.

Part of the reason "Tuvix" is hard is because, at least superficially, Tuvix appears mentally competent, so it's much harder to justify ignoring his wishes.

(As a second aside, one thing that always surprises me about the Tuvix Discourse™ is how little attention is paid to Tuvix being... well, somewhere between an asshole and a creep. He is incredibly manipulative toward Kes, preying on both her feelings for Neelix as well as her mentoring relationship with Tuvok. Neelix of course always was a bit possessive and jealous, but he at least was written "with a good heart"; I felt like Tuvix took those same qualities, but added a Tuvok-esque cold calculation to it. In any case, to me there's a mildly interesting parallel between these two episodes where there's this tone-deafness to the way the writers treat the behavior of the "transporter accident individual.")