Because of your statement, the phrase "a murder of opportunity" - which was never actually used by RJ! - "more than implies that the killer did NOT plan it." This is completely wrong.

Means, motive and opportunity are typical criterias in any murder, irrespective of the question if the murder was planned or not. It's also false that if opportunity is the most important criteria in a specific murder, it must mean that this murder wasn't planned at all, but just some kind of unplanned accident or chance.

To get a better understanding of what we are actually talking about, you should read the Wikipedia article about it [link below].

I cite:
"In US Criminal law, means, motive, and opportunity is a popular cultural summation of the three aspects of a crime needed to convince a jury of guilt in a criminal proceeding. Respectively, they refer to: the ability of the defendant to commit the crime (means), the reason the defendant had to commit the crime (motive), and whether or not the defendant had the opportunity to commit the crime (opportunity)."

And the corresponding example:
"If a criminal shot someone with a handgun and took his/her money when the victim was in an isolated, secluded area at night, the means would be the handgun, the motive financial (i.e., the money they stole), and the opportunity the fact that it would be unlikely someone else would witness or stop them."

So, the mentioning of opportunity as being important by RJ doesn't tell us at all that the murder wasn't planned.

As said before, some people just claim that RJ's usage of the term "road kill" would prove that the murder was basically an accident. This is wrong as well, though, since we don't know the context in which RJ mentioned this. It's at least as likely that RJ just wanted to paraphrase his reasons as an author to take the redundant character Asmo from the story with "road kill". Otherwise, if RJ actually wanted to give such a major clue to the murder, it could even have the meaning of "political road kill", which would indicate that Asmo was killed on the orders of one of his Forsaken rivals.