Is there anything wrong with saying negative things in the form of an exaggeration, or as a manner of speech, when one doesn’t really literally mean what he’s saying? For example, “I’ll go crazy if she tells my mother”, or “When you die, your soul goes to heaven” (suggesting the person being addressed will die)?
The custom is to avoid saying such things. This is often referred to as “opening one’s mouth to or for Satan”.
What we refer to as the uniquely human “power of speech”, from a Jewish point of view, is just that – a power.
A misuse or abuse of our power of speech may be harmful to ourselves and others. This is because we are linked through our bodies via the soul to the spiritual realm, which is replete with various forces. Things we do, say and even think, have an effect on this realm for good or for bad.
Just as something we do may have a ripple effect or set into motion a chain of events in the physical world, so too things we say impact and ripple through the spiritual plane. The forces thus set into motion ultimately rebound back into the physical plane, much as ripples rebound from the far edge of a pond back to where the pebble was thrown.
In this way, negative speech may boomerang back, and therefore should be avoided.
Thus, in the examples you bring, a person should rather say, “I’ll be very upset if she tells my mother” (avoiding extreme exaggeration), or “When one dies, his soul goes to heaven” (avoiding the colloquial “you”).
What’s more, we find in the Torah that Rachel died in childbirth with Benjamin (Gen. 35:16-20). There is an idea that this may have been a result of her having named her previous son Joseph (where ‘yosef’ means to add), “saying, ‘may G-d add me another son” (Gen. 30:24), referring to Benjamin. Her saying it in this fatalistic way is understood to be a possible cause of her demise.
Accordingly, a person should avoid saying even positive things if it’s in a fatalistic context. For example, one should not say, “If I get that job (or get married or have children or see my children married, etc.) I’ll fulfill my purpose in life”, since putting it this way limits the purpose of one’s life to the attainment of the goal. Rather, one should say something like, “If I get that job etc., it will be one of the greatest moments in my life, and G-d willing, just the beginning of many great contributions…”