Morality comes from us - we make it. It starts with a few biological sources, such as the basic survival instinct, and behaviors of social species. After that, human philosophy, and assessment of harm versus benefit, establish the remaining nuances of morality.

Atheists use this "Secular" morality explicitly, which is basically a derived set of behaviors that allow multiple humans to cooperate in a society. The goals are to maximize benefit and happiness while minimizing harm and misery (which biology primes us to desire). Actions taken can be evaluated towards this goal as moral and immoral, respectively.

How do we know not to kill people? Societies break down fairly fast if people run around slaughtering each other. It does not take long to figure out that this is a behavior that gets in the way of society - thus, it is determined to be 'bad'. It is reality based, not supernatural.

It is like a family trying to sit down at the dinner table to eat. The reason that it is bad to chew with your mouth open is not because it was declared in some book, but because it disgusts the other members at the table, to the point that they are less willing to eat dinner with you. Thus, chewing with one's mouth open is deemed wrong.

Another analogy is the rules of the road (and by "rules of the road", we are not necessarily talking about strictly-enforced laws, but more a common agreement and understanding among drivers). Many people make use of the public roads to their advantage, but if we don't have a common set of rules that helps facilitate convenience, while minimizing accidents/casualties, traffic turns into dog-eat-dog chaos. The rules exist because they objectively improve the situation, not merely because some ultimate book said so.

Each action can be assessed as to how much harm vs. benefit it generates. Thus, a rule set of morality can be derived from the patterns of human behavior.

If humans wish to survive, typically, we need to cooperatively live together. That is a biological imperative, driven by the evolution of a social species, and our basic survival instincts. We accomplish more as a whole than as individuals. Without cooperation, society wouldn't be able to specialize into experts like scientists, police and farmers. Instead, we'd all be mediocre at the same small sets of skills, individually trying to survive. In order to cooperatively operate as a group, we need to figure out how to not step on each others' toes. Morality is the assessment as to what helps towards that goal, and what detracts from that goal.

No supernatural being required.

Whether one agrees with this argument for non-religious morality, the important point to walk away with is that atheists are moral people as much as theists are - regardless of where one thinks that morality comes from.