Ah, comma splices… How I hate them. If lazy/(willfully) ignorant writers didn’t do comma splices so often, there would be enough commas available for use in compound sentences. As it is, there’s a comma shortage — that’s the only possible explanation commas missing from compound sentences, right? — due to so many writers adding commas where they flat-out don’t belong or using commas in place of periods. It’s similar to the apostrophe-shortage problem noted by Lynn Truss, author of Eats, Shoots, & Leaves.
Remove the comma after identity and replace it with a period.
Now take that comma you removed and place it after was.
She couldn’t let them know her true identity. If they knew who she was, they would be sure to kill her.