We cannot deny that murdering our eldest brother was an enjoyable experience. Raziel, while never openly bragging or claiming to be our superior, still behaved arrogantly enough to make us hate him the way those without power hate those with. We may have been brothers, but we were far from equals.
I suppose it is ironic that when Raziel first appeared with those beautiful wings, the first emotions we felt were jealousy and envy. Showing them off the way he did, arriving at the Pillars and spreading them for us to see, did little to lessen our bitterness and resentment. Raziel was the first-born, the best fighter, and probably Kain's favorite, so why did evolution grant him all the advantages? Why was one of us not given such a gift to parade in front of our Master? The irony, of course, was that the wings were no gift. As we all soon found out.
And so murdering him had two justifications. First and foremost, to destroy our superior and blood rival and perhaps place us higher in Kain's esteem. And second, to punish him for his arrogance and stupidity. Being granted wings was just his bad luck, we felt. We weren't envious of them anymore. They were Raziel's gift, let him suffer the consequences of their existence.
Of course, we had not exactly expected Kain to react as he did, damning Raziel to a watery grave like that, but it soon became apparent that we could not anticipate anything that our Master did anymore. Raziel's execution was just the first link in a chain of events that none of us could have predicted, especially not then. For a time, with Raziel out of the way, we all felt like we were in charge of our destiny, subordinate only to Kain's power. But it did not take long for us to realize that we had control over nothing. In a way, we were just like Raziel, doomed to fate and the whims of our own evolution. It just took longer for us to find this out. I suppose Raziel might have been the lucky one after all.