I think Stannis is the bigger traitor for thinking he has some claim to the iron throne just because his brother killed the previous guy who was sitting on it. That he killed Renly makes him a traitor to his family, too. He is also a traitor to the old Gods.
Renly would have killed Stannis too, given the chance. He was actually more of a traitor, since he had no reasonable claim to the Throne. At least Robert had Targaryen blood and overthrew a deranged King, and at least Stannis was Robert's legitimate heir under all laws of succession.