According to the preamble, the purpose of the declaration is to break up the bond between the King and the colonies because the King as a ruler and as a form of government has not been able to protect or has even purposefully breached the unalienable rights for his own good and against the people.
The three unalienable rights that are listed are life, liberty, and the pursuit of happiness. They are self-evident and unalienable meaning that nobody can or should try to take them away from you and you are born with those rights. This is based on the philosophy of John Locke.
According to Jefferson, the primary reason for the existence of a government is to protect these rights. If a government is not protecting these unalienable rights then it should be changed for a new one which is what they want to do to the British crown since it is not protecting them.
According to the declaration, the government derives power from the people who choose it. It is specifically stated that the power of the government comes from the consent of the people to be governed. If the people don't want a government they are allowed to change it.
It is appropriate for the people to abolish their government when the government becomes tyrannical and breaches these unalienable rights and stops protecting people who elected it. This is why he states grievances as to what the king did to hurt the people which is why he should be changed.
So they (the slaves) were treated bad because they weren't considered human beings
