system
Given the following post, respond as if you're commenting on Reddit:
human
[Half Life/Black Mesa] Why did HECU bother killing valuable scientists and such to ‘cover up’ the investigation, and why do the marines even obey illegal orders like that so easily?

It just doesn’t come across as a realistic response. The Black Mesa facility is full of VALUABLE scientists. These people aren’t just easily replaced. Surely a more realistic response would’ve been to rescue everyone and get them to sign non-disclosure agreements? People have kept top government secrets. I mean, presumably these guys kept their work secret from the public before the resonance cascade. You’re much more likely to have someone leak something if you try to kill them or get them to commit atrocities and war crimes. Surely the marines now know what went on? How are they any different from the scientists? The scientists aren’t any more likely to leak info than the marine is.

It just seems like a huge waste of human resources, guys that have valuable knowledge and experience. I can’t imagine there’s a huge supply of top scientists ready to replace them instantly. There’s obviously tons of them in the facility.

Not only that, I imagine a large majority of the soldiers wouldn’t want to kill fellow US citizens right?

It just seems like an unrealistic military response considering the factors at play. And yes the alien invasion isn’t realistic but people are real and when they don’t act in a realistic manner it’s weird.
gpt
Black Mesa had been experimenting in inter-dimensional travel, including the construction of a functioning teleporter and contacting extra-dimensional, sapient life. Should they have gone rogue, they would have presented an existential threat not just to the United States, but to humanity as a whole. That's why the government was prepared to annihilate the facility and liquidate its personnel with a hair-trigger; it would be better to ensure any knowledge of how to access Xen be wiped out than allow potential rogue actors or turncoats for alien powers to walk. 

Of course, this contingency would never be sprung so long as Black Mesa kept their teleportation under control, and never allowed a breach in containment that could signify an invasion.

As for HECU, its pretty clear from some of their speeches and actions that they have a disregard for human life, and were probably selected with this in mind. They aren't your regular-joe soldiers, they're a spec ops force being used to liquidate a government black-site potentially full of extra-dimensional monsters. They were likely created with the specific goal of carrying out 'illegal orders', given how many ethical and legal boundaries both the government and BM had already crossed long before this point.