The first murder occured when Adam and Eve sinned. Satan was the first manslayer, he caused Adam and Eve to bring sin into the world and in turn death. Also Cain and Abel gave sacrifices because they felt a need for God's favor. Cain presented "some fruits", it was not said to be of the choicest fruits whereas Abel's was specified as "firstlings of his flock, even their fatty pieces". Abel clearly gave his best. Abel's sacrifice was accepted over Cain's also because of motive, as you can read in the bible cain killed his brother even after God warned him to change his ways. After being rejected his motives came to the surface. If Cain's motive was to please God he would have listened, humbled himself and imitated his brother, but he did not. Hebrews 11:4 and 1 John 3:12 clearly tell each of their motives.

What a deep, simple ending! Excellent! Technically though, it appears that Cain was the oldest son (though we don't know that) and we know for sure that Seth was born after Abel died. Great story, though.