The Thing doesn't really want to be humanity, it wants to destroy it via infecting it, like a disease, and replace it with itself.
I'm not sure that viruses "want" to destroy their hosts. In fact, if I'm not mistaken, the most successful viruses are the ones that don't destroy their hosts. Take the human rhinovirus for example: one reason why it's so successful is that it actually does minimal damage to human beings once it infects them. That way, there are always more hosts to infect.
I prefer the "eldritch horror" interpretation advanced above. The Things might want to destroy us--if they gave us any thought. But they're completely indifferent to our lives and deaths, and consume human beings the same way that human beings consume air, water, and plants. To the Things, the violent resistance they encounter from potential hosts has no ethical significance: it's just an obstacle they have to overcome in order to stay alive, the same way that human beings have to shield themselves from the cold of Antarctica.