Black people have free reign and ownership of the modern use the N-word. True, we cannot impede your right to spew hate speech, however, you cannot predict or control the reactions of those you refer to as the N-word.
The relationship Black people have with the N-word is complicated, historically and presently. Every Black person's relationship with the word is different. Some Black people would like to do away with the word completely, as they find its use offensive no matter who says it. There are some Black people who find the word carries no significance anymore and can be said by anyone. There are Black people who believe a careful selected few of close friends or loved ones who aren't Black can say the word.
One of those two have told you can say the N-word with no consequence, just between us.
Here's why you shouldn't say it if you're not Black: you're not Black. It's as simple as that, your exclusion from the community means that you aren't able to take ownership of a racial slur that would never even be used against you. You would never experience this word in the negative ways that a lot of Black people still do.
This principle applies to any marginalized group that has reclaimed slurs, but take offense to those outside the community using the word. I would not use the F-word because I am not a homosexual male who can understand what it feels like to referred to in such a derogatory way. Long, painful histories attached to reclaimed slurs remain despite their use by the communities they're meant to offend.
The loaded history of the word still remains because of that it's a marked difference in what I feel when a Black person versus a non-Black person says “nigga." You can remove that hard -er, but the negative connotations and racism remain. You are an Other in my community, and when you use the word that fact is immediately heightened.
I can assume without question that even if a Black person I do not calls me the N-word that day it with no malicious intent. They say cannot be assumed when the word is used by someone who isn't Black. It immediately takes back on its historic meaning.
Black people have taken a racial slur and robbed it of its destructive power if it's used within the community. However, it's history hasn't erased, so no matter the circumstance universal use of the N-word will never be okay.