How big of a sin is killing of insects?

Question

I have created a separate question for killing of animals as in case of animals , one can easily avoid killing of animals. It is one's choice whether or not one kills an animal or not.

While in case of insects , it is almost impossible not a kill an insect in one's lifetime either directly or indirectly. Indirectly we kill insects by consuming agricultural products or living in house whose construction causes death of many insects.

Directly most of us would have killed insects when it bothers us or when we walk without looking at the ground. Doing these things involves killing of insects.

Second reason why I created a separate question for insects is that in modern science it is debatable topic whether or not insects feel pain as they don't have a brain.

Answer 1

There might be a possible answer in the Śrīmad-Bhāgavatam Purāṇa

Canto 5: The Creative Impetus » CHAPTER TWENTY-SIX

ŚB 5.26.17

यस्त्विह वै भूतानामीश्वरोपकल्पितवृत्तीनामविविक्तपरव्यथानां स्वयं पुरुषोपकल्पितवृत्तिर्विविक्तपरव्यथो व्यथामाचरति स परत्रान्धकूपे तदभिद्रोहेण निपतति तत्र हासौ तैर्जन्तुभि: पशुमृगपक्षिसरीसृपैर्मशकयूकामत्कुणमक्षिकादिभिर्ये के चाभिद्रुग्धास्तै: सर्वतोऽभिद्रुह्यमाणस्तमसि विहतनिद्रानिर्वृतिरलब्धावस्थान: परिक्रामति यथा कुशरीरे जीव: ॥ १७ ॥

Translation

By the arrangement of the Supreme Lord, low-grade living beings like bugs and mosquitoes suck the blood of human beings and other animals. Such insignificant creatures are unaware that their bites are painful to the human being. However, first-class human beings — brāhmaṇas, kṣatriyas and vaiśyas — are developed in consciousness, and therefore they know how painful it is to be killed. A human being endowed with knowledge certainly commits sin if he kills or torments insignificant creatures, who have no discrimination. The Supreme Lord punishes such a man by putting him into the hell known as Andhakūpa, where he is attacked by all the birds and beasts, reptiles, mosquitoes, lice, worms, flies, and any other creatures he tormented during his life. They attack him from all sides, robbing him of the pleasure of sleep. Unable to rest, he constantly wanders about in the darkness. Thus in Andhakūpa his suffering is just like that of a creature in the lower species.

If one consciously kills an insect, then that will amount to a sin (big or small whatsoever), but indeed a sin.

In the above verses, the sin of killing an insect seems quite grave with an horrendous description of the resultant hells for the sinner.