yes, it's possible to buy a car that will go 250mph and is legal however neither you or I or 99% of the population could afford to buy one. Even if we bought a used one we couldn't afford the maint and insurance. People can always build a bomb or crash an airplane or find some other inventive way to do mayhem but using these means is way more involved and difficult and there is way more of a chance you will be found out before you can accomplish your aim. Buying a few guns and some body armor and ammo is something anyone with a credit card can do in minutes and in a few days you can just walk into a mall and start firing away.