Atheists cannot appeal to religious reasons to justify their actions. Without religious commandments to guide them, Atheists must formulate their own personal reasons for how they exist in this world. How they choose to behave becomes a matter of their own reflections on what is the right thing to do in a situation. It comes from their mental judgments and their hearts. So, yes, I believe it is possible for them to be more benevolent if they so choose, because whatever they choose, it is a matter of actual choice and not some outer authority telling them what they ought to do. It is easier to follow rules than it is to formulate them on your own, and when you are merely following rules it is easier to break them. It is much harder to break a commitment you have made to yourself.
I think that being generous or benevolent is not specific to being religious or not, I think it's a decision every human has to make personally if they feel like being generous, so I do think someone who is an atheist could easily be benevolent if they chose to be so.
Atheists are fully capable of being benevolent human beings. They do not believe in a supreme being or God, but that does not mean they can not be kind to other people. They would not choose to involve themselves in benevolent projects or actions involving religion or churches, but could definitely be involved in charity work that was of the more secular nature. There are many secular programs that help many people - not all are religion-based. Atheism is a belief based on what their mind believes to be true - that there is no God. It does not mean they cannot still act with their hearts in other areas.
I have often heard the religious man ask- "without god, what is to stop you from going out and raping and murdering people?" Personally, i'm more worried about the followers of imaginary beings suddenly deciding to go convert the unbeliever by fire and sword... historically speaking, i know which is more likely.
I think it is possible to be more benevolent if somebody is an Atheist. But I do not think that it means they will be. I think that it really depends on the person. It doesn't matter if a person is an Atheist or a Christian, everybody has the possibility to be more benevolent.