I mean, this question is not just about normal criminals.
Think like very bad crimes. Like serial killers, rapists, child rapists, terrorists, corrupt officials, terrible leaders, cruel dictators, generals that ignore laws of war, or like people has bad as Hitler. Which of these people do you think deserve a respectful burial, if any.
Is there a level of evilness that you think should not be allowed to have a proper buriel or have their corpses mutilated. Or should everyone deserve a respectful burial regardless of crimes.
I personally don’t even know how to answer this question myself. Like the funeral isn’t even for the dead. Its for the living. So to me, the question seems like, should the relatives of a bad person be allowed to see the corpse treated respectfully. I personally don’t have an answer to this question.
One good way to answer your question is to ask the exact opposite question: why wouldn’t they deserve decent funeral?
How would it help anyone to refuse decent funerals to a dead body? No matter their crime, the ‘real bad guy’ is now dead and is no more. It’s a body not a person anymore.
And then, one may want to consider this question: why would anyone want to punish innocent people (the family of the ‘real bad guy’ has committed no crime, right?) by refusing them the right to pay respect to the deceased? And if it is somehow right to punish the family for crimes they have not committed, have they been (secretly and silently) trialed? By whom and for what crime exactly? And who was their defendant?
One may also want to question their desire to hate so much on a person as to hate their corpse and then, once again, to apply their hate onto innocent people, aka the family of the ‘real bad guy’.
I generally agree, but there are two addenda.
First: Even the worst should be burried with dignity, because their behaviour is not the standard by which we measure our actions. Nobody is so evil that they can take our will to be decent human beings. So we do the right thing (decent burrial) to spite evil.
Second: With dignity is not the same as “with reverence” or “with honor”. In many cultures criminals are denied certain parts of funeral rites (like processions, official or acknowledged mourning periods). This reinforces social norms to the living (don’t do the bad thing or you will be shunned by society) and can also prevent retraumatizing their victims. The most common form of this is not allowing to have their gravesite marked. This is done so that their family may have a place of grief (the unmarked grave) but to prevent the grave from becoming either a shrine to their followers or a target of defilement by the victims. A fairly well known example of the last part is Adolf Hitler who was properly buried in an unknown location and then a parking lot was put over the area with the possible grave sites.