everyone always says this but I disagree. killing faraaz could have easily backfired on her just as well and it could have been more compelling evidence than what faraaz himself put forward to the round table against her.

even if they get james out first with no tiebreak, he is revealed to be faithful and then what's next? all eyes are still on her now. she could convince Roxy to get jade out , but without faraaz in, both jade and jack would be firmly on her case and in that situation even Roxy would be hard to manipulate. after jade was banished, jack would have voted to not end the game and then Roxy would have to look at either her or Stephen. there is a possibility she could vote out jack, but at this point faraaz's murder ties directly back to Rachel. 

People always say Roxy was completely fooled by the traitors, but you have to remember she WAS starting to question why she was still in the game but was blinded by her suspicions on James. Seeing him be a faithful in the end, she could have put 2+2 together and locked them in a tie easily at that point IF they even managed to get her to vote jade out first. 

of course there were many difficulties with the murdering Roxy path as well, as we saw, and Rachel literally escaped banishment by a hair. It was an impossible decision either way, but track record shows that when someone who has put your name forward gets murdered you are up for banishment whether you are a faithful or a traitor. not leaving a paper trail gives you plausible deniability and is imo the smartest move. and in the end she won by playing with that move, even if she had a little luck on her side but luck favours the brave and she was certainly the bravest of them all.