He can block them from ever be spend for example:)
Go for a win-win: team-up with him and double (triple, quadruple, n-uple...) spend at will
It still requires clever engineering to actually profit from double spend attacks, even with 90% hashrate under control, you can't force every node on the network (thinking of the exchanges) to think and accept whatever you would like them to.
Yes, but the more value something has the more likely such a think will be.
That was generally spoken, feeleep wouldn't do that!
And me neither, since (and i know everyone on the internet says that;-) i'm a honest person.
But as told earlier it's more important for the future and the integrity of the coin itself, although i'm not sure if there will be a future, since i don't like some of the ways it is now hyped "to da moon", and if that won't do some non fixable damage.