...why do you believe that?

I think I explained why I believe that animals should have a right to their life in my previous reply when I said "parting from the premise that every being wants to keep living for as long as it possibly can". I've never seen any living thing that wants to die therefore it's safe to assume that it wants to keep its life and what right do I have to take their life if I can choose?

Do you think that taking a human life should have the same weight as killing a bacterium

This is an extreme comparison of course, bacteria are not complex organisms like the animals humans breed for our own pleasure. As I said, I don't see it as a question of value of a bacterium vs a human, but a question of necessity.

We decide to put subjective human value on living organisms at own own peril. Just like most structures in human society, we've been trained to think about nature as a hierarchical food chain (with us on top, anthropocentric much?), when it is in fact a web of life.

Every living thing is valuable in its own unique way. That does not mean that we should kill everyone with equal dismissal. Quite the opposite, it means that we should treat all life with equal respect. Yes, including bacteria if we can choose to. For example, I choose to garden organically because I want to promote insects and living organisms in the soil and not kill everything with pesticides, herbicides, fungicides and chemical fertilizers.

Of course we can't always prevent killing micro-organisms or even small insects but the question of morality does not arise in circumstances where you don't have the knowledge or choice. As a supposedly evolved human being, in a supposedly advanced and abundant society I no longer need to subvert, breed and take the life other beings for survival, therefore I choose not to. Veganism is about reducing the amount of cruelty and suffering even if it might not be possible to eliminate it completely.