Machine Elf, if you hadn’t mentioned the Pacific Time zone and an “apartment” I’d have thought you were my own brother! All other details match up perfectly. It’s also possible you have slightly changed the details to protect the guilty. 
I am a trader and also derive income from various other sources which would be deemed to be not contributing to society. Deemed that way normally by hippies who use the word “community” every other sentence and who wouldn’t know what to do with themselves if they had to look out for their own family alone in the country for a few years.
To which I say: I don’t owe anybody anything, except those individuals that I do owe. Certainly not society. I pay my taxes. If I didn’t earn an income and pay taxes on it, the money would have to come from somewhere else. And where do you suppose that money comes from in the first place? A lot of of the money I “earn” comes from individuals residing in countries that aren’t exactly friendly to us, if you know what I mean. I can’t see how that’s a bad thing.
But I understand you are only asking about the trading aspect of this. (Even though you were the one who brought up your father talking about contributing something to society… perhaps some great job like flipping burgers?) So about the trading:
First what your brother is doing likely does not qualify as HFT. But I’m not there, I don’t know if he’s got a co-located machine, etc. Just saying.
Day traders can provide liquidity and that is never a bad thing. As just_some_guy5 said, most of them lose money. The stock market is not a zero-sum game. Even options are not exactly zero sum. To qualify as a “parasite” he’d have to be taking money from somebody else and offering nothing in return – I don’t see how the term fits here. If he’s got the intelligence and skills to make money for 12 years straight without having to work another job then you should be happy for him.
Hey, it’s not for everyone. What I don’t get are the people that think it’s somehow better to be flipping burgers or working in a coffee shop. Car payment, mortgage, and sometimes dying in the elderly years with a quite possibly negative net worth. How is that a good life? I’m tempted to agree with Kimmy here.