Salam. Is it permissible to call a Muslim a bastard, because they transgressed their right?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Calling a Muslim with such a name is not permissible, and it is an evil speech and accusation in one’s honor.

Ibn Qudaamah said: "If a man says: o son of adultery (produced as a result of adultery), or son of an adulteress, then he is accusing his mother of adultery. If she is alive, then it is an accusation to her and not to him, because it applies to her [..] if his mother is dead, then it is a slander to him, as it is a defamation to his family lineage." [End of quote]

The fact that he had transgressed the rights of others does not justify calling him with such a name. Rather, he should be advised about his transgression in a soft and gentle manner, and the bad consequences of injustice should be clarified to him, and that it will be darkness on the Day of Resurrection, and that he must repent from it.

Finally, it should be mentioned that a Muslim should train his tongue to say good words; he should either say good words or keep silent.

Many people enter Hellfire because of their tongue. Mu'aath narrated that the Prophet said: "What will cause people to be thrown onto their faces in Hellfire except the harvest of their tongues?" [At-Tirmithi]

Allah knows best.