The number one trait that I hate is selfishness.
I truly despise selfish people. Selfish in the sense of a disregard for other's feelings or well being in order to satisfy ones own comfort. I've been around a lot of people who act like this. It seems their rationale is that being selfish over the little things in life, is okay because when it comes to the time to be selfless, (as in- when it's important- or- for the big things in life) they believe they have the clarity and intention to do so. It's the small decisions in life that matter, because they add up to a lot. People like this don't see that.