Once a thief always a thief...

I have heard like this once upon a time Mannar Thirumalai naicker who caught  a thief but he won't punish him. Instead of punishment the king who give the duty to the thief to protect the precious things. The king think like this the thief is not always a thief.

We have our judgement and prejudices clouding our thoughts about a person who has been found guilty of some wrong doings in the past. Old habits die hard and it takes a really tough decision and will to come out of it. There are many who fail and remain a thief throughout. But there are many who undergo reforms and change genuinely. It also needs a good mentor, guide or guru to lovingly nurture you and bring you to the right path. Sometimes inner realization works like that Guru.

Everyone deserves a second chance in life. So do thieves who really wish to reform. Not to forget the great story of Daku Unglimaal. 

I agree but with a suggestion. It's not the case with more than 50% people who do it in most challenging conditions but when they think about the consequences then they reconsider and mend their ways. But, I agree some who find it lucrative get involved deeper.

It could be very unfair to judge anybody because they could be forced into such actions due to circumstances. Everybody deserves a second chance.

For small thieves this rule is applicable but not for politicians. Where I live in Maharashtra, a lady whose father was a politician has accumulated a lot of land by land grabbing and she will generally not pay her servants on time or will pay less. But still people respect her. 

Corrupt practice is generally not a compulsion but, mostly an attitude. So, a person taking an unfair course once is frequently destined to take unfair means in later stages too since, after you get involved in corruption once, you may try your level best to abandon corruption but, corruption never abandons you.  

If a person is not asked to pay for his crimes there are many chances that he will do it again. A penalty or punishment will definitely make him think twice before repeating the same mistake.

Depends on situation