When we don’t even know who the killer is or their motivations

You're right, we don't, and obviously the right have created a narative saying it's a leftwinger. The point is not to provide them a ready made narative. Killing kirk will do little to nothing to improve matters, much like killing that CEO has done little to nothing. What it does do it make it seem more "reasonable" for there to be crackdowns, after all there are "crazed lefties" on the loose killing innocent people. (Yes, it was hard typing that, which doesn't change the fact it's effective propaganda, even if it was a fase flag)

The only people the “pretext” works on already support fascism.

Fascists will obviouslt support it, but many ow information people, or even just people who are worried about how they'll manage can be easily swayed to accept harsher measures with stories like this. After all, they're not assasinating people, and assassinating people is bad, so if "the left" are doing it, they must be bad, and maybe they should be dealt with like that. As I said, it's propaganda, and making it easier for them by playing into it is probably a mistake.

It’s up to us to stand up to fascism, not avoid angering the abuser.

Go ahead and anger the abuser all you want, all you can, and stick in an extra kick for me. It's not about them though, it's about "hearts and minds", cliched phrase though that is. Individual assasinatons, unless they're of key personnel, whose elimination can actually stop something happening, will achieve little positive, and create a convincing story that it's just evil lefties killing people they can't beat any other way.