Telling the uncomfortable truth plainly is a reward, a grace to someone. Why would give this favour to a bad employer? Don't lie but let them sort out why you (and other valuable people) left. Find a friend to vent to.
Because that's not compassion. The mercies we offer are chances for redemption. My team lead was young and inexperienced. I hope, with wisdom, he learns that telling the truth maximizes good outcomes.
When the compassion & effort of providing the info looks like it may possibly have a good effect, and is likely to be reasonably safe (e.g., minimal retribution/fallout effects, and is delivered with kindness - it can be good for all.
But one should first consider the consequences. Sometimes it really is best to just get away while the getting away is good.
