BARGAIN --BOOMERANGS

The" Karna" of  the real  Mahabharata written by Vyaasdev is different from the stories  made up  by his followers. 
Let us look into the incident of his  giving the kavaca and kundala to Indradeva.

Karna had taken a vow to kill Arjuna by all means -- in fact he  had vowed  that  till he killed Arjuna
1. he would keep  donating  to the brahamanas, 
2.never eat anything that  was produced in  water, 
3.never allow any one to wash his feet.  
So it was a  very clear goal that he had in his mind---  killing Arjuna.

When Indra came to Karna, Karna was almost convinced that he could not  kill Arjuna and at the same time Arjuna could not pierce his armor.  
 
It would be a  tie --  a match where neither would win
but in his passion to kill Arjuna, Karna was looking for some thing which would help him kill his sworn enemy.  
Indra gave him that chance.  
He said  he would make Karna  famous in history  as the  giver of everything, including protection of his very life "the kavaca."

Karna was scheming; "give the kavaca which any way does not kill Arjuna but gives me great fame, and at the  same time ask  for something which has the power to kill Arjuna.”  

 For killing Arjuna he had to take  the risk of becoming defeatable. 
It was a gamble  but he was willing to accept the gamble, 
great fame of being charitable and at the same time a chance to kill the sworn enemy.  The proposal within his own mind was very exciting.  Of course this was not openly mentioned by Indra.

The gamble proved disastrous for Karna,  for he lost his life being brutally killed by Arjuna, though  he got some fame for being charitable amongst those who admired him .

Strategising is good, but  strategy needs to  be shrewd and not deceptive.  
Because Dharma is  a greater strategist.  Dharma will pounce over us and be victorious.  
Ultimately it was Indra who represented  God’s administrative potency, which fooled Karna, and Karna fell for it and died.