“The beginning...the end.” This is a polarmerismos (giving the two extremes to include the whole). The “beginning” and the “end” are the two extremes, and by juxtaposing them with each other Ecclesiastes 10:13 is saying that everything that the fool says is hurtful foolishness (which is a hyperbole, but makes the point).
“hurtful.” The Hebrew is “evil,” but the way the Hebrew uses “evil” it often does not have a moral quality, but means more like “hurtful” and refers to disaster or harm. That is the case here.
“end of his mouth.” Here, “mouth” is put by metonymy for that which comes from the mouth, which is words. We have this kind of saying in English, for example, when we say, “Watch your mouth,” meaning watch what you say.