I used to view freedom as a license to do pretty much anything that wasn't illegal or unethical. If someone else was bothered by it, it was their freedom to be bothered or not bothered by whatever they chose. They could choose to react differently to it; there was no requirement for them to react by taking offense. Until the shoe was on the other foot. That's actually how I came to the Stack Exchange.
I had spent a lot of time at one of the big computer forums. One day, not that long after the 9/11 attacks, I noticed the avatar of a new member. That member was from a country that actively supports terrorism, and where people grow up marinated in an ideology of hate. The user's avatar was the slogan of a particular terrorist organization, preaching extinction of another culture.
I had a pretty visceral negative reaction to it and complained that it was hate speech, and that the forum was providing a free platform for dissemination of that message. The site owners decided that it was free speech. The new member claimed I was a nut case for being offended by a harmless slogan they saw everywhere, every day, espousing an idea everyone they knew agreed with. I quit the site.
Freedom of expression isn't a simple concept. There are words and language that everyone recognizes as offensive. There are other words and language that offend some people and not others. The people who aren't offended often view the people who are as being unreasonably sensitive. I learned first-hand that what's reasonable or unreasonable is very much in the eye of the beholder, and others may not understand that from their own perspective.
With that context, let me address this question. Does the desired username and avatar violate the site rules? Is it offensive to some users but not prohibited if there is an assumption of good intent? People can argue all that, and someone with authority can rule on it. But let me appeal to the user temporarily known as "user310650".
Jaydles's answer explains why this particular username is offensive to a portion of the community. Now you know, and the ball is in your court. You may have the right to use that username, it is obviously important to you and you have nothing but good intentions. But it is also your choice whether to exercise that right.
It doesn't need to be a zero-sum game where one side wins and the other loses. Please consider voluntarily looking for a compromise username that's close enough to satisfy your own needs without being offensive to a segment of the community. Thanks.