Is their any reason to have morals within such an existence? Any reason to keep your feral impulses in check?
An excellent question, and one which all philosophers since the beginning of time have struggled with. I think one of the "benefits" of religion is that it keeps us from having to ask (and attempt to answer) this very difficult question (and other questions like it). With a supernatural being around to put everything into clear categories of right and wrong, we can rest easy not having to think about such things ourselves. It's really easy to see how religion developed as a result of the uncertainties of human existence. With science and reason to cast light upon our ignorance, the notion of God no longer had the power to keep our minds at ease anymore. And so Nietzsche told us: "God is dead"--and he was right.
Anyway, here's my own take:
Is there really any reason to be moral, to keep our impulses in check? Possibly not, which is why society has evolved as a force to keep such things in check for us. Since we recognize that others' impulses can harm us, we recognize the value of society despite having our own impulses limited by it. So it's easy to see why, when some people decide to take their own lives, they have no problem with taking out a bunch of other people just for the hell of it because they have no punishment to fear from society if they are killing themselves anyway.
But the way I see it (and this is where it gets really deep), all existence seems to be tied together in ways unimaginable, which should give pause to anyone thinking about inflicting harm upon another without good reason. What's to say there is any real separation between, say, my existence and your existence? The only real thing separating our individual existences is the perception by both of us that we exist individually. Take away that perception and what are we left with? Well...
We have a physical world in which the atoms and subatomic particles that make up our bodies are constantly intermingling, have been part of other things before we were around, and will continue to be part of other things after we are dead (think: laws of conservation of matter/energy). So there's physical interconnection.
We have a mental/spiritual world from which our consciousness dwells, and which seems to be highly connected to the physical world if not a part of it entirely. We understand even less about this then we do about the physical world, but again, there seems to be interconnection.
Interconnectedness implies to me that there is a oneness to existence that goes beyond our individual lives and the (temporary?) perceptions life binds us to. Perhaps all existence is one in the same: a massive jumble of perception of which we only have partial access to at any given time. So if you do harm or cause undue suffering to someone, it might only be another facet of your own existence to which you are causing pain.
In any case, from being on the receiving end of pain, I feel that the mere knowledge that there is probably some entity feeling the pain I cause to be reason enough to try not to cause any. The thought that this entity could be myself only reinforces this feeling for me.