There are no time paradoxes. If you go back and kill your grandfather, you don't suddenly cease to exist. Hillson could very well have succeeded in killing astronauts Shepard and Mitchell and possibly even the command module pilot, Roosa, and in destroying the Apollo program. The people in every thread deserve to be safe from maniacs from another. It doesn't mean what Hillson was trying to do wasn't dangerous or even fatal to people in another thread, and like a fine cloth, these threads are all intertwined in some way.
Events can branch and spin out of control, so one never can be sure.
It's best to play it safe and have a care. Hillson's actions were wrong, plain and simple. We have a responsibility when playing with time not to pollute it. Len heard the spook grunt. Sounds right to me.