Part of what makes Conrad's work so compelling is that it forces the reader to carefully examine and assess characterizations to a point where it becomes difficult to see them as entirely "one thing" or another. Arsat falls into this distinction. There is enough in the story to reveal him as both a lover and a betrayer, and it might be here where Conrad is suggesting that to be human is to possess the capacity to be both. Conrad develops his main character as one that is "only human." He loves and betrays, and like all human beings, both realities exist in his heart.
If one were to make the case for Arsat being a betrayer, the abandonment of his brother at the most critical instant is the most compelling justification. Further detail here would involve how Arsat betrayed social conventions in coveting another man's woman. Yet, it is here where Arsat is a lover and where I think that there is more substantiation to see him as one. Arsat pledges himself entirely to Diamelen, reflecting love in an absence of betrayal. When she dies, he pledges to go back and fight for his brother's honor, reflecting again love where betrayal might have been. I think that much of this is going to depend on the reader, whom Conrad has constructed as one who is is more prone to see the ambiguities and moments of doubt and ambivalence in what it means to be human. There is little absolute in Arsat, and this goes to the core of whether or not he is a person who loves or a person who betrays. Perhaps, this is Conrad's ultimate statement about what it means to be a human being.