OK, I found the "complete and uncut" at the library.
The character who claims the scientists were all dead, that it killed them pretty quickly, actually had no way of determining that. He bolted as soon as the code red came up. He should have been locked in with all the others, but they explain the emergency locking mechanism didn't work as fast is it was supposed to, and he just barely made it out. He bolted as soon as the red emergency warning came on. There was no way for him to know anyone's condition. He never checked, he just ran. So, you can dismiss his assumption it was "pretty quick". It could be he's exaggerating to light a fire under his wife so she'll leave instantly with him, or it could be he saw some dead people on his way out and assumed the virus had just gotten free right before the alarm sounded. King doesn't seem to explain that claim, that I can see.
We know from all the subsequent cases described it takes about 12 hours to show symptoms, and death came about 2-3 days later. There's no telling how long the disease had been free in the compound before the scientists realized it and sounded the alarm. Obviously, since he's the one who spreads it to the world, the guy who escaped had already been infected before the alarm sounded. It took him something like 3-4 days to die, and he kept driving till he lost control and crashed.
The guards who didn't try to get out, I would assume, were doing their job and mindful of the fact that if they left they'd just be contaminating the whole world. They were going to die anyway. No point in killing other people as well, or in letting other infected people escape to do so. I don't see the fact they died at their posts as proving they died suddenly.
I didn't re-read the whole book, of course, but there's no explanation in the first 4 chapters of how anyone in the compound got infected to start with, how the virus got from test tube to human host. And, hence, we have no knowledge of the time lag between that happening and the sounding of the alarm. King is more concerned with the story of how it got from the lab to the general population. The character, Campion, does seem to think it killed the others just about instantaneously, but that's obviously something he assumed, not something he witnessed.