I see there are multiple opinions on the matter. I have my own, and for the time being I will say this (these are my opinions):
1. Every chapter has its own ingrained personality and thus, their own opinions of the human race. I have identified (in my opinion) 4 categories of Space Marine mentality with respect to humanity.
a. *Protective*: Some chapters feel that humans are the source and future of the Empire. Humanity is the reason they are doing what they do. They [Marines] are "created" by the Emperor from humans, to "protect" humanity. Are the individual marines able to relate directly to an individual human? Probably not. The space marines are so far above and removed from human desires and connections that cannot relate to them at all. But they still value them.
b. *Paternal*: Humans are usually incapable of knowing what is best for them. They are like children in the eyes of the Marines. The typical space Marine, as we know, is far more intelligent, wise, and certainly physically superior to any human. So much so, that they are gods unto the human race. But even gods must still repect the lives and needs of those they rule. Humans may be ignorant, but they are still living, sentient beings with souls who worship the emperor (misguided though that may be). Thus, all humanity is worth protecting and guiding to greater glory in the service of the Emperor and the Empire of Humanity.
c. *Indifferent*: Some chapters feel that humans are beneath them, but humanity has value. The mass of humanity (as a whole) has value; it produces, it repairs, it toils, and it grows. The Human race (as a whole) is the Empire. An individual human, has a duty to the Empire. Sometimes an individual must give up his/her needs for the group. The marine knows that humans are necessary, and valuable commodities. They will not go out of their way to harm humans without cause. They will however, not be above sacrificing an individual, a group of humans, or even an entire planet for the greater good, or to fulfill an important objective. There needs to be justifiable cause, of course.
d. *Scornful*: Some chapters feel that humans are useful, yet disposable (as one writer put it, as humans->squirrels). The mass of humanity has its purpose and place. The Space Marines of this mentality place no value on individual humans. They are a resource, necessary for the continuance of the empire, a labor force, a tool. However, a single human is no more valuable than a single nail in a box. There are more to replace it.
-This does not, however, insulate them from the subsequent taint and corruption of their souls resulting from acts wanton cruelty and neglect.
2. One must also keep in mind that an individual marine, has his own personailty, opinions, thoughts and desires. It is a given that he will be heavily influenced by his training, indoctrination, and the over-arching influence of the primarch's gene seed he carries (thus gaining much of his primarchs personaility). Even this will not wipe out all traces of an individual personality. If this were the case, all Marines would merely be autonomous clones of their primarch and have no (true) free will of their own. This is simply not true.