Or another answer: terror tactics or just destroying anything big enough to group people, which, in my view, is the most likely possibility. It has been confirmed that there are a large number of schools across the country that have been destroyed, the "mistake" or the "false flag" options are just not realistic.
That's it and there are at least 2 reasons for that.
1. The west hit Russian citizens by banning certain goods and services so that these citizens put pressure on Putin, so he decided to do the same thing and put pressure on Zelensky by killing his people.
He thinks that civilian casualties may break the fighting spirit of the defenders.
2. Ukrainian soldiers are hiding in basements, they need to house wounded, store ammunition, missile launchers. More people killed in night bombings means less equipment lost the next day.