Theory:

     “If you insist on calling him a criminal, I don’t think I want to discuss anything with you, Maya,” said Nishad angrily. “He can’t be such a bad man if he gives Ramesh such generous tips.”

     “Ramesh probably knows something about his past, so Mr Nath must be bribing him to keep quiet,” I said.

     Nishad glared at me with his arms tightly crossed across his chest. I was beginning to get fed up with him.

     “How can we make any progress with our investigations if you take that attitude, Seven?” I asked.

     “I’ll cooperate only if you give up this idea about him being an escaped crook,” said Seven. “You really make me angry.”

     I almost hit him. “I make you angry, you stupid oaf,” I shouted. “You make me mad! What is the point of all these enquiries if he’s not a crook? If you think he’s a nobody, what’s the idea of bothering about him, please tell me?”

     Nishad looked thoughtful. “I’d like to find out why he’s so thin and why he’s so lonely. I want to know why he doesn’t have any friends and lives alone.”
Explanation:
 
Nishad became enraged and informed Maya that if she continued to claim that Mr Nath was a criminal, he would never be a part of the discussion anymore. He went on to say that Mr Nath couldn't be such a terrible guy as he always gave Ramesh such big tips.
 
Maya had another amazing idea after that. She responded to Nishad's claim by suggesting that Ramesh might be aware of Mr Nath's past. As a result, the tips from Mr Nath might be a bribe to keep Ramesh from disclosing the truth.
 
Listening to Maya's baseless suggestions, Nishad became angry. He glared at Maya with his arms tightly crossed across his chest.
 
On the other hand, Nishad's response and attitude were getting on Maya's nerves. She was irritated and questioned Nishad about how they were supposed to progress with their investigation if he kept acting like this.
 
Nishad then declared that he would cooperate only when she would abandon the notion that Mr Nath was a wanted criminal. Like Maya, he too was firm in his belief and wanted his sister to acknowledge and adopt his idea that Mr Nath was innocent and poor. He later adds that Maya's attitude towards Mr Nath was upsetting and that she had been irritating him greatly.
 
The irritation was mutual, for his sister responded by stating that he was the one who had been irritating her.
 
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Nishad and Maya began arguing
 
She then questioned him why they were doing the investigation against Mr Nath when Nishad was against the belief that he was not an escaped criminal.

The question seems appropriate because, though it is clear to the readers why Maya was investigating, Nishad's purpose remained a mystery. He never believed that Mr Nath was a criminal, but the question is, why did he go along with the investigation?
 
Nishad looked thoughtful, and he pondered over the question. Then he said that he was doing the investigation because he was curious to know why Mr Nath was thin and lonely.
 
Through the investigation, Nishad desired to understand why Mr Nath didn't have any friends, looked poor,  and lived alone.
  
Meanings of difficult words from the paragraphs:
  
Sl. No.
Words
Meanings
1
InsistTo state or demand forcefully, esp. despite opposition
2
GenerousShowing a readiness to give more of something than is usual or necessary
3
BribeTo try to make someone do something for you by giving them money, presents, or something else that they want
4
Glare
To stare in an angry or fierce way
5
Fed upTo feel annoyed, unhappy, or bored, especially with a situation that has existed for a long time
6
Attitude
A settled way of thinking or feeling about something
7
Give UpTo stop doing something, or to stop making an effort; admit defeat
8
OafA person who is rough or clumsy and unintelligent
Reference:
National Council of Educational Research and Training (2007). Honeycomb. Expert Detectives:Sharada Dwivedi (pp. 126-132). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.