No, it is NOT illegal. It may violate the license agreement, and you could be sued in civil court for that, but it is not a crime.

It all depends on what the license says, *exactly*. If the license for the HL SDK prohibits commercial use, then you''re fine. Why? Because you haven''t sold anything yet. It''s still just a hobby or a free mod until you actually sell it. As long as the infringing code is out by that time, you are still complying with the license.

Besides, do you really think Sierra is going to waste their time on this? At most, their law-bots will send you a nasty letter asking you to...remove the infringing code, and not do it again.

Still...this is shaky ground at best, and you would probably be best off to:
a) not do it,
b) not talk about it if you do it, and
c) talk to a lawyer, who will simply repeat all of this advice, but for $180/hour.