The girl was "2 weeks pregnant"? She probably DIDN'T EVEN KNOW SHE WAS PREGNANT.

I'm guessing the marine sensed that the situation was grave enough that running wasn't going to be enough. He probably took out the knife (which slowed him down) because he deduced that they would continue to pursue him (he was on foot, they had a CAR).

It's too bad he didn't manage to kill the other 3 attackers in the process.

Since one was maimed, that means the marine stopped attacking him once he was taken down. The story doesn't say, but did the marine then run or did the attackers run?