Because they were preparing as a trap to Ukrainian soldiers for when they had to fight inside the city. There are also examples and reports from north of Kiev and the city around it of them booby-trapping closets, pantries and other places. So when the Ukrainian soldier opens the door, or triggers the trap they are either killed or maimed.

But, the killing of civilians by Russians makes sense. We just need to look at how they wage war(and the incident with their equipment, remember Kursk?) to see why they would act the way they do. If they don't care too much about the lives of their own soldiers, why would they care about the lives of civilians of the "enemy"?

EDIT: Some examples of booby traps https://twitter.com/ralee85/status/1511782023555342337

Because they were preparing as a trap to Ukrainian soldiers for when they had to fight inside the city. There are also examples and reports from north of Kiev and the city around it of them booby-trapping closets, pantries and other places. So when the Ukrainian soldier opens the door, or triggers the trap they are either killed or maimed.

But, the killing of civilians by Russians makes sense. We just need to look at how they wage war(and the incident with their equipment, remember Kursk?) to see why they would act the way they do. If they don't care too much about the lives of their own soldiers, why would they care about the lives of civilians of the "enemy"?