I would say probably not.
So technically speaking, what makes the creation of Horcruxes possible is the act of tearing your own soul. (killing is just the way you tear your soul)
However, in the same book when we see Snape's memory of his conversation with Dumbledore, Dumbledore makes the following comment after asking Snape to kill him:
"You [Severus] alone know whether it will harm your soul to help an
old man avoid pain and humiliation" (Harry Potter and the Deathly Hallows Scholastic hardcover edition page 683)
While granting the standard disclaimer about whether Dumbledore was lying or not, it does imply that one can kill without tearing one's soul. That being said, the conditions under which one might not tear their soul are not clearly defined, but at least from what Dumbledore says, it seems the motivation behind killing plays a part in it.
In this case, the question becomes "Would Molly tear her soul when killing Bellatrix?" If so, then yes, she could make a Horcrux, given the knowledge and desire (though I would point out that given what we know of Molly, she wouldn't make a Horcrux, even if she had the ability to do so).
If the answer is no, then she couldn't make one, but the question becomes why wouldn't her soul be torn? In Molly's case, her killing of Bellatrix was motivated by love for Ginny. And given that love (especially a mother's love) is portrayed as being a powerful protective magic under certain circumstances that may protect Molly's soul from being torn.