अयुक्तः प्राकृतः स्तब्धः शठो नैष्कृतिकोऽलसः ।
विषादी दीर्घसूत्री च कर्ता तामस उच्यते ।।

ayukta: prākṛta: stabdha: śaṭho naiṣkṛtiko’lasa: |
viṣādī dīrgha-sūtrī ca kartā tāmasa ucyate || 18.28 ||

Tāmasika kartā

Krishna uses very powerful words to describe the tāmasika performer:

Having no self-control and piety, vulgar, obstinate, fraudulent, given to robbing and allied habits, slothful, morose and always postponing matters – these are the characteristics of the tāmasika kartā.

The tāmasika performer miserably lacks inner control and moderation. He does not have any regard for honesty and other human virtues. Far from being cultured and refined, he is vulgar, gross and arrogant. He does not mind being even deceitful.

No sense of gratefulness or regard for anyone or the society occurs to him. He will not mind denying or even robbing others of what is due to them. He is lazy by nature. He is despondent and likes that kind of mental framework. Instead of being prompt and sensitive to matters, he is given to postponement and delays.

Sāttvika traits are generally promotional in character. Tāmasika traits are demotional. And the rājasika features are extreme, given to possessiveness and domination.

By entering into a discussion of this kind, Krishna emphasizes the art and process of dealing with our mind, intelligence and ego. It is something that an intelligent human should necessarily know about. Our body completes its growth in the first twenty years or so of life on this earth. Thereupon it is the sole task of the mind and intelligence to grow, imbibe enrichment and be as best a human model as possible. Spiritual discussions are meant only for highlighting this need for attaining excellence and fulfillment.

Again, the discussion is not to make the individual evaluate or pass judgement on the members of the society. The greater purpose of the discussion is to enable each examine his own personality and know where he stands and what his option should be.