She had used a knife to kill William because he was a man. As a woman, she was at a disadvantage in raw strength, so to ensure he was taken out instantly, she chose a blade.

As for killing William's wife, she used a chokehold and suffocated her instead—she was confident she could finish the job that way, and using a knife would have caused blood to spray everywhere, splattering onto her body.

In this apocalypse, it would be difficult to find water to bathe. So she chose the more exhausting me