No substance is always good or bad in the world. Sometimes some substances are felt good or bad relatively.
That thorn is considered good which is used for removing the other thorn but if it pricks, it is felt to be bad. Sweets are really tasty but become bitter or distasteful to the patient of gall fever. Cold substances or cool air is unfavourable in winter while favourable in summer. If poison is used properly, then it can become medicine or food can also become poison if it is indigested. If misused merit or virtue is also the cause of hell whereas if there is good use then merits can be earned or attained even in the rise of sin karmas. So learn to use everything so that life may become successful and meaningful.