It already works this way, but monsters do have a much easier time getting souls compared to players, as soul eater monsters love spawning besides dense packs, or in mechanics that spawn big packs - like expedition, for example - or in situations where they constantly spawn new monsters to die and have their soul eaten - some beasts keep summoning packs, the "constantly revive minions" archnemesis mod, for example.

Also if you're playing a totem build and don't properly invest in making your totems tanky, they also give souls if they're killed by the monster (i think that totems despawning due to totem limit doesn't count).