So what is the value if life based upon?
Size? Intelligence? Looks?
Why is it that 99% of people would feel next to nothing for killing a fly. But at the same time feel extremely guilty for killing a cat?
They're both living creatures, they're both one life. What's the difference?
When people discover an ant nest on their property, they may attempt to destroy/kill the ants without a second thought.
Pouring boiling water down holes causing the suffering of hundreds of ants. Yet most people don't care.
If they had a cage full of dogs and had to pour boiling water on them until they died it would be a entirely different story.
I think our current society bases the value of life mainly on how much we empathise with, or are attracted to the animal in question (maybe how human it looks?). Polar bears can look quite cute, and they do stuff we recognise - playing, hunting for stuff, swimming etc. Jellyfish, however are totally alien to us, and don't look at all familiar or human, and many people find them quite scary and repulsive. Also, nobody I know floats around underwater waiting for food to swim into their tentacle things, which makes it quite hard to empathise with them. Imo that's we don't see many 'save the jellyfish' campaigns, but we see loads of polar bear ones. (I don't actually know if jellyfish will benefit or lose out due to climate change, but it seems like a good example).
I think actually valuing a life on a purely objective, non emotional basis we might think mainly about an animals capacity for emotions, or creativity, or whatever (maybe puzzle solving ability? - But the type of puzzle would bias some species over others, so this would be hard). However, not only would gauging if an animal had any of these attributes would be extremely difficult, if not impossible, but these are still things which we, as humans would think of as valuable, but the majority of life on earth may not.
So in fact, 'properly' valuing life is probably impossible, and going with the 'dogs and cats are more valuable than an entire ant colony' way of thinking seems like the only realistic course of action*. Maybe just acting 'naturally' is the best thing - kill what you want and don't kill stuff you can't/don't want to kill. After all, humans are an evolved form of animal, and it was natural selection that produced us.
*to avoid getting flamed by society( - Nobody cares if you dump an ant colony into a bin), and also because putting e.g. ants in front of cats would probably make more people angry than the other way round, and we do live in a democracy after all, even if the members of the democracy aren't quite sure what it is that makes them feel something, they should be able to have their 'natural' desires embodied (in the form of animal cruelty legislation) if they want them to be.