Having bad thoughts or bad words thoughts about parents will Allah punish such person ? Is it a sin?
I had bad thoughts about parents too.What I did was first of all tell clean jokes.Ive heard the saying of the prophet telling a joke to make an old lady happy.Also understand your path in life can be easy,difficult or somewhere in between.If you have a debt owed to parents and cannot even say a single word,ask allah to settle your debt with your parents.Allah or trying your best are two ways i found to pay back parents what they deserve. Also following each of allah's commands is best if you try your best. I used to get wrongly scolded and parents used to change their mind.I forgave them but I became wise.
In Islam we are expected to respect our parents. This does not mean that we are to expect that our parents are perfect - and nor ourselves. Hence its quite possible to have disputes between you and your parents. In which case asking for the help of a family friend to help mediate the dispute can be helpful.
Of course if someone is pretending to be your parent, then one ought to categorically reject that pretence. The only situation where that is justified is where this is stipulated by law. For example, in British law school-children whilst at school are under the protection of the school, and the school acts in lieu of the parent as in loco parentis. Here of course, there is no pretence - it's merely a legal term that describes a certain type of guardianship in analogy to what ought to be the case.
The question then is whether your 'bad thoughts' are justified. That you used the term 'bad' suggests not. In which case you should with them as with all thoughts that originate from Shaitan - reject the whispering that darkens the soul and persevere in purifying ones's heart.
Of course it goes without saying, that if someone is pretending to be your parent and is in a position of power over you (and without any legal standing) then it is justified to have 'bad thoughts'. But this would call for stronger medicine than merely bad thoughts - one would expect that under those circumstances that the both the law and the police ought to be involved.
A different example, but nevertheless an important one is the way the current Chinese administration in China is handling their Uighur muslim minority. No doubt they consider their intervention as 'paternalistic' in 'educating' a minority out of their way of life, given that political communism/capitalism is atheistic and that they would see this as the best way of life. Nevertheless, it's ethically wrong and not at all paternalistic - one would normally describe this as brain-washing rather than 'education'. In such a case, one would be justified in having 'bad thoughts' about the so-called paternalistic Chinese state.