User blog comment:HazelQuill7445/RWBY Volume 4 Dead Pool/@comment-27404492-20161003203543/@comment-28288412-20161003203854

I've always reckoned that Cinder was a bit more careful with Mercury, not trying her luck due to him already having the guts to fight and kill someone stronger than him (Marcus).

With her newfound power she could very well rub him the wrong way and trigger him to try and kill her. What Emerald would do in that situation is anyone's guess.

Regardless, when they become obsolete and no longer useful, she will probably kill them.