moodhagraahenaatmano yatpeedayaa kriyate tapaha |
parasyotsaadanaartham vaa tattaamasamudaahritam || 19 ||

That which is performed out of a foolish notion, causing pain to oneself, or for ruining others, that penance is called taamasic.
moodhagraahena : foolish notion
aatmanaha : self
yat : that which
peedayaa : causing pain
kriyate : perform
tapaha : penance
parasya : other
utsaadanaartham : for ruining others
vaa : or
tat : that
taamasam : taamasic
udaahritam : is called
The story of a any captured terrorist is all too familiar. At some point in their life, they start holding on to an idea or a notion which ends up becoming their mission in life. They then spend a long time, sometime years, learning all kinds of tactics and techniques that are usually reserved for the military. Eventually, they carry out their mission, in which a great deal of harm is caused to others, and to themselves as well. Either they get captured or they harm themselves in the process.
Shri Krishna describes taamasic penance in this shloka. He says that penance based on a foolish notion, a misconception or an infatuation comes out of the minds of taamasic individuals. The end goal of such penance is to cause great harm to others or to oneself. We can always refer to the Puraanas for the fate of individuals who conduct severe penance just to bring about the downfall of someone else. So called “black magic” is also conducted for harming someone.
We may look at this and dismiss it, since it sounds so extreme and not applicable to our daily lives. But many of us sometimes perform actions with a malicious intent. Politics sometimes becomes a venue for one party to perform actions simply for bringing the other party down, and not for the welfare of the country. Similarly, many people run businesses based on a personal vendetta. For instance, a person fired from a business may join a competitor just to get back. Any action performed for bringing someone else down usually backfires.