I think it's a semicolon. Not sure, though.

This essay is due tomorrow, and while I really don't care too much about one semicolon/comma, my super grammar editor has failed me, and I really wanna know the answer.

Besides the fact that the sentence seems ambiguous in that "him" could refer to the murderer or the son of the murdered logically, I believe that a comma would work (especially if it was phrased "... he would not just be taking this man's life, but that he would also be robbing..."). As it is, however, I think a semicolon works best because it reinforces the first clause (complete thought on its own) while also standing as a complete idea itself. Two complete thoughts with the latter embellishing the former makes for appropriate semicolon usage I believe.

Ok dumb that down for me.

It can't be a comma as it is. You can make it a semi-colon or split into two different sentences. (man's life. He would be...)

i hate seperating sentences, as evidenced by my obsessive comma use.

So semicolon is the way to go?

Yep! Thanks Bobo

Thanks guys!

vamosrammstein, you have gotten good advice here. You seem to like what is called the splice comma and the general rule is that you can only do that if you're a famous writer; the rest of us must use a semicolon to indicate our mediocre status.