I agree that neutrals can be self-interested, but they are not supposed to be too callous or totally blind to the well-being of others. They certainly don't have to go out of their way to help someone, but I think murdering someone for gear is tending slightly towards the evil side of the scale.
I'm not saying a neutral can never kill someone for gear, but I would hope that they didn't make a habit out of it. Pure selfishness is a clear trait of evil, and as the helpfile says:
Heedless of the well-being of others, evils selfishly and shamelessly
exploit others for their own advancement and power.*
I can certainly see a neutral person looting items from a corpse they found without any problem. I can also see a neutral killing someone for gear, if they didn't do it all the time, or if there were some other underlying reasons.
While neutrals can be selfish, once their selfishness extends to having no compunction about killing anyone for gear whenever they feel like it, I think they are slipping into evil.
The neutral alignment enjoys several benefits. You can hunt with both evils and goods, you are not affected by protection from evil or good, and good aligned characters need a decent reason to attack and kill you. The price of this is that people expect neutral align characters to act with reason that isn't totally motivated by self interest. Although as I said, it is usually OK in moderation.
These are just my opinions.
