Seems excessively pointless and petty for a creature that powerful. Is he a low-budget anime villain now?
Given his lengthy speech at the final confrontation, I'm inclined to answer "Always has been".
Jokes aside, I wrote the actual suggestion quickly, and used Tchort taunting the player as a simple example of some sort of hostile interaction with the player character. The main point of the suggestion is not to specify *how* Tchort should antagonize the player, but *that* Tchort should antagonize the player. You mention that he's a psionic demigod trying to kill everyone who approaches except high ranking Tchortists, but that's something that needs to be established before it can used as motivation.
You're correct, that the game does provide good reasons to want Tchort dead. He sends hordes of respawning monsters after you, he has a legion of brainwashed followers attempting to continue Biocorp's mistakes, and if the Faceless are to be believed, he is a soup of Biocorp scientists, who are responsible for hideous experiments and set the stage for the current geopolitical conflicts of Underrail.
Here's the problem: This information is found by exploring the environment. An environment you can't explore until *after* you are tasked with killing Tchort. In short, you find out why you should do something *after* you are instructed to do so. It's completely backwards, and why the quest comes across as contrived. Indeed, prior to entering DC, you aren't even certain that Tchort exists; you only have the Tchortists' word for it that it does, and cults aren't know for being unbiased. Sure, like me, most will roll with it because the choices are either kill Tchort or stop playing, but that doesn't make the narrative any better.
As for Tchort's motivation for the contact, the natural answer would be curiosity, or perhaps territoriality. The player is a new presence in Tchort's domain, and Tchort would only expect Eidein or his trusted guards to enter from that elevator, so when the player arrives, Tchort might want to investigate. It could be a good moment to revisit your roleplaying decisions. If you were an obedient Tchortist, he might be willing to let you go until he finds out you're after the Cube. If that suggestion sounds like the player character is being treated as too important, remember that there is currently a dialogue option where the character feels like a chosen one and IIRC Six doesn't deny it. It might not be great, but the writing wouldn't be getting worse.
Now that I think about it, another way of getting the player on board with the killing Tchort agenda would be to force the player through Hollow Earth for a little bit, getting attacked by Tchortlings, and end up meeting Six by the mutagen gates, where he can launch into his exposition dump. That way, the environment can do the storytelling, which is Underrail's strong suit. It would require more work though.
