Pete... to fully answer you as to why it is NOT consistent to consider humans as no better than mosquitoes... etc, it would be necessary to send you to the very learned writing of various Philosophers... such as Immanuel Kant in "The Fundamental Principles of the Metaphysic of Morals".... but that gets way too thick & technical.
The closest I can come is to note analyses such as this, where explanations that resemble religious ones are noted, but without specifically positing a 'god'. You get formulations like the Golden Rule which make sense because we ARE an unusual species, no matter what you attribute our character to.

It simply is NOT the case that accepting evolution can justify euthanasia for the aged...and if one (you gotta do better that "I heard that") makes such a silly claim, it is no different than one Christian theologian asserting some astounding nonsense.