Why is unintentionally but knowingly killing a bunch of innocent civilians better than intentionally killing evil leaders?

Cluster munitions are banned by most countries, but the United States and Ukraine did not sign that ban.

And neither did Russia

The difference will be where and why they are used. Ukraine needs them to attack fortified trenches (the reason the US has them) and Russia has used them as a terror weapon against civilian targets and going as far as to paint the submunitions to look like toys to kill children that find the unexploded ones.

Context matters

While it would be great to stop the need for these weapons, there will always be bad actors that will be willing to slaughter millions mostly due to ego.