The only thing is that salt inserted into the rectum will cause the bowels to absorbs water from the rest of the body; and in doing so, it loosens and expels as much fecal matter as possible (depending upon several factors, of course.)
I mean, is there a danger in stimulating a hellacious bowel movement during a suicide attempt? Maybe not? I have no idea, tbh. We're all going to experience the evacuation of our bladders and bowels when we die anyway. But I know from experience that a saline enema can be quite a terrible undertaking. Ugh. And the concentration of salt to water ratio used for an SN attempt: oof. That crap is gonna burn your poor little butthole, dude.