A slightly separate issue for landmines is the tendency to main but not necessarily kill. It might seem morbid, a lot of the logic revolved around whether a weapon caused unnecessary pain compared to quick death. Dum Dum bullets for example banned for military use for their tendency to cause more pain than needed (sadly only applies to military). I believe before that there was similar debate on a sword type used by cavalry.

Coupled with its indiscriminate nature, and denial of swaths of lands, and it's an cruel atrocity of a weapon.

Automated weapons I feel fall under something else.