Doing the Math
"She did the math," Mal mutters some time after the crew have all wandered off to leave him to his thoughts. "Why is that not a thing of comfort?" He clasps his hands together and looks studiously at the pattern in the wood of the table. The pointless focusing on it allows him to open his mind and further explore the specifics of River's newfound psychic powers and even newer inhuman ability with the use of weapons.
She poses danger. That, in the end, is what it comes down to. She is a weapon and an unpredictable one at that. There is no telling when she might lash out or who she might attack. There is no way to know what will set her off or if she will do more than just inflict pain on whoever falls victim to her. There is no way if she will snap and prove fatal to one of them.
It's a huge risk keeping her on board when she can, without warning, turn on them and attack at any one of them. She can endanger them on heists too or put their employers in a situation where future jobs they have, they will direct to someone else – assuming they live beyond the situation River poses them. They will direct their jobs to someone safer. Someone less unpredictable. And he knows they can't afford that. They need food on the table and they need to keep Serenity in the air.
But River is family now. Simon, too.
As things go, family is important to hold onto. Family, whether related by blood or not, are the only ones there when you need them. What will it say about him if he kicks them off? What will the crew think? Alternatively, if he keeps them on, will that be a mistake? Will something trigger River to hurt or kill them all? Will they be of use or be too big a burden on the crew?
Mal takes a swig of alcohol from the flask Jayne left behind on the table. He sets the flask back down and exhales loudly. A moment later when the silence gets to be too much, he stands and heads for his bunk. Maybe inspiration will hit and supply him with an answer in the morning.