Since no one had been killed/murdered prior to Cain killing Abel, the concept of murder must not have been known/understood. Did Cain actually realize that by striking Abel, that he was going to die? The intent of murder couldnt have been in Cains mind since the concept was not known. Am I off base with my viewpoint?

There is such an opinion found in the Oral tradition (Breishit Rabba 22, 12) and in the Radak (one of the classic commentaries, about 800 years ago), and it's definitely a legitimate viewpoint to say that Cain didn't realize that he was killing Abel. Nevertheless, it's not definite, for the other opinions would say that the Cain could have and should have understood the finality of his action from seeing murder and preying in the animal world. Additionally, the first blow probably did not kill Abel, and Cain should have had mercy after seeing the painful results, not to mention that he surely had experienced pain himself from falling or different blows he received during his childhood.