In Greasy Lake, what is the protagonist striving to acheive?
In short, he wants to be a "bad character." He wants to rebel and taste the freedom that comes from rejecting (at least momentarily) morality and any sense of societal pressure. He wants to be what, at least in his mind, Bobby and biker (the corpse in the lake) represent--people who live according to their own will and desires.
However, of course, when the narrator does taste of this freedom, it proves not to be as glorious and fulfilling as the narrator imagines. The act of hitting Bobby with the fire iron and attempting to rape Bobby's acquaintance bring him face-to-face with the realities of the consequences of such behavior: death, symbolized by the corpse in the waters of the lake.
From this, the narrator learns that this "bad character" way of life is not what he once imagined. And, at the end of the story, when presented the opportunity to take drugs with the other two women who appear at the lake, the narrator very consciously make a different decision than he did the night before.