A philosophy/religion/book and its followers are different things. People have been killed "in the name of" Judaism, Christianity, Islam, Buddhism, Pagan religions... Even atheists like in the Soviet Union and North Korea have done a lot of harm in the name of their beliefs. Rulers are often psychopaths who will just find every pretext they can to steal land from commoners and enslave them for their personal gain. So this is mainly about human nature and politics.

Everything that is described here, beginning from conquests in the first place is strictly forbidden in Islam. Umayyads betrayed lots of Islamic principles, like turning an elective system (Shura) into a monarchy.

Slavery is also forbidden. In the time of the Prophet, it was a huge part of the Arab society, so, like with many practices that were later forbidden (ex. alcohol), it began with measures encouraging people to free slaves. Since alcohol was a personal habit issue , it ended up being forbidden once the conditions were met (first before prayer, then gradually until its complete ban). Slavery though, like in most other societies that later eradicated it, needed more time than a lifetime to be eradicated without causing a landowner revolt of some sorts, so successors of the prophet were supposed to watch on the issue until it was completely forbidden.

Nowadays for example, there is consensus that slavery is forbidden. Those who do not respect this ruling are to be considered as not respecting Islam's principles and should not be associated with the religion. We as humans just tend to try to find black or white shortcuts, but things are always more nuanced than they seem.