Mankind

Islam itself does not require certain beliefs about scientific matters, as long as one's scientific beliefs are compatible with the Qur'an.

The Qur'an speaks about the creation of Adam from clay. Many Muslims take that in a literal sense (Allah fashioned Adam from clay the way a person might make a statue).

However, some Muslims understand that verse in a way which is compatible with evolutionary theory, and that Allah used the evolution of species as a means to produce the human being from what was, originally, the earth.

There are some other viewpoints which are somewhat intermediate; for instance, the view that Allah created other homonids through evolution, and then intervened when it came to our forefather Adam to create or change him specifically.

Of course, from a scientific perspective, the theory of evolution as it applies to human beings is also a developing idea and may be revised or replaced in the future.

So, basically, there is no single view on this topic at present, and a Muslim can hold any idea as long as it is compatible with the basic ideas of the Qur'an (for instance, that Allah is the ultimate creator and had some intent behind the creation of Adam), and as long as they are trying to find out what is truest.

Some people say it is because Allah had created the jinn first, and the angels had seen the jinn fighting so knew that a new species with free will would also fight.

Some say that there were other humanoids (nasnas), and they had fought, and so they deduced from this that a new species with free will would also fight.

These two views are reinforced by a narration from Imam al-Sadiq (A) in which he says that the angels could not have known that humans would cause bloodshed if they hadn't seen the precedent of earlier creatures on earth.

Some simply say that they understood this from looking at human nature, or by looking at the nature of life on earth and the challenges of living as an earth-based being (for instance, competition for food, water, reproduction, and land).

Since the earth (dunya) is considered the basest or lowest level of creation, vis-a-vis the heavenly realms, perhaps it also didn't make sense to instill Allah's vicegerency in an earthling. 

Maybe there are other reasons too which relate to the nature of the angels and their existence outside of the earthly realm which would give them access to knowledge or a perspective that human beings generally lack.