Do you not think the karma of killing a small insect is less than the karma of contributing to the death of a human?
I do not know. Life is life, correct? Is it correct to value some lives over others? If all is part of Brahman—as is the self—then doesn't harming insects harm myself? Are there degrees of value for lifeforms? Do different sects have different opinions on this?
Most importantly, if we say that different forms of life have different value, then we are participating in separation and distinction. How can we value things differently and be one with reality at the same time?
There are some streams of thought connected to rebirth which indicate higher value of humans when they believe that bad Karma will result in you being reborn as a lower life form.
There are some like Advaita where God and all creations are one and the same. This inherently implies all creations are God and thus have same value. No hierarchy.
In Sankara's Advaita on the other hand he believed in the authority of Vedas and they indicate a hierarchy. Other Bhakti gurus have believed only in what is humane even if scriptures say otherwise.
In Hinduism there is usually no one truth. There are multiple paths and they all eventually lead to same path if you are always trying to be the best you can. Different scriptures give different opinions in the same topic.
So just believe what feels right to you. Anyone who claims to be an expert or that there is only one truth is misguided.
 According to Vivekachudamani
2. For all beings a human birth is difficult to obtain, more so is a male body; rarer than that is Brahmanahood; rarer still is the attachment to the path of Vedic religion; higher than this is erudition in the scriptures; discrimination between the Self and not-Self, Realisation, and continuing in a state of identity with Brahman - these come next in order. (This kind of) Mukti (Liberation) is not to be attained except through the well-earned merits of a hundred crore of births.
Yes, human life is considered to be of higher value because of the possibility of a human attaining moksha.
The swan [Brahman] said, "I disclose unto you a great mystery. There is no status that is superior to that of humanity. Freed from sin like the Moon from the murky clouds, the man of wisdom, shining in resplendence, attains to success by patiently waiting for his time. A person of restrained soul, who becomes the object of adoration with all by becoming the foremost of the supporting pillars of the universe, and towards whom only agreeable words are spoken by all, attains to the companionship of the deities."
Can the unity of all existence postulated by Advaita Vedanta coexist with a hierarchy?
The answer is yes. The unity of existence is only true in the non-dual state where there is no other being and hence no hierarchy. It is a philosophical position. We live in a dualistic world where we do not see the unity. For example, bacteria, fierce animals and men threaten human lives and may have to be killed. So there is a hierarchy in the dualistic world from the practical point of view.
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