True, because looks weird in that. But you wouldn't even need to have it there. "I'm definitely going to kill that guy, he did kill my family, after all."
Granted, that uses TWO (ZOMG!) commas, but it would be still look better than adding a because. A dash would still look better in that case, though.
Here, we run into an issue I don't point out in my comments, because it is such a technical one... However, I always did mark it on student papers, because it is one of the "7 Deadly Grammar Sins." The example is a comma splice:
"I'm definitely going to kill that guy, he did kill my family, after all."
That is two independent clauses joined by a comma. There are 2 fixes- (1.) Use a coordinating conjunction with the comma, (2.) Use a semicolon.
I see a great many comma splices in the fan-fiction section, even among some of the best writers. How to tell if it is a splice:
Is each element (on either side of the comma) a complete sentence? If yes, you need a coordinating conjunction and comma OR a semicolon.
Example:
"My goal was clear, the broccoli must die!" WRONG
"My goal was clear; the broccoli must die!" CORRECT
There is not a really good way to use a coordinating conjunction in that example, so here is another:
"I went to the store, the dog stayed home." WRONG
"I went to the store,
and the dog stayed home." CORRECT
As to dashes, I overuse them, without any real reference to the rules for correct usage. (Even though I have 3 or 4 style manuals and grammar handbooks near to hand.)