A bad kind of playing is when you tease or prank someone. It's mean and you shouldn't do it! Depending on the context this can sometimes have a far more negative connotation and mean lewd behavior.

The reading for this word is an exception. Here's a mnemonic to help you remember it:

If you tease someone with a prank that makes them eat zoo rats (いたずら), you're a jerk. That's less of a prank and more like abuse. Cut it out.