When combined, 变质 (biànzhì) literally means “to change quality.” Since it's not specified whether the change is for the better or worse, the cultural context has firmly established it as an exclusively negative transformation.
The concept of 变质 (biànzhì) taps into a deep-seated cultural awareness of change and the importance of maintaining integrity, or 质 (zhì). While every culture understands that food spoils, the frequent application of 变质 to human character and relationships is notable in Chinese. In Western cultures, one might say a person “lost their way” or “became corrupt.” These phrases often imply an external influence or a series of bad choices. 变质, however, feels more internal. It suggests that the person's fundamental substance—their 质 (zhì)—has rotten from the inside. This reflects a cultural perspective, influenced by philosophies like Confucianism, where maintaining one's moral character is a core responsibility. To say someone has 变质了 (biànzhì le) is a severe condemnation, implying they have failed not just in their actions, but in preserving their own good nature. It's not just that they did a bad thing; it's that they became a bad thing.
变质 is a common word used in various contexts, from the kitchen to serious moral discussions. Its connotation is always negative.