Well, given how unimaginative a leap the national socialists took from the national fumigation program to the gas chambers, I doubt there was much more to it than "take bug poison room tech, use it on humans".
There's no such thing as an "engineer" in the real world, and there's almost always somebody more unscrupulous, devout, or patriotic than you who knows or is willing to learn what it takes to finish the job, and school is not going to get in the way.
Added: Engineering is a practice, not an immutable identity. Even if you publicly shame all the "engineers" away from contributing to an effort you disagree with, replacing them is a matter of learning enough of the practice to complete the task. Even if we were just talking about what is in paper books today, it's hard to say that anyone could have control over who is or isn't capable of engineering efforts.