Warcraft 3 was made by Blizzard back when they were a respectable company. Back then Blizzard was VERY meticulous about balance, polishing, and fixing any bugs with in their games. Now, 7 years later, the game has been patched so much that I highly doubt it would cause any trouble at all on any platform.
Warcraft 3 will not slow down any Macbook Pro that has ever had a 13" screen, the specs are simply ridiculously overkill for that game. That said, you must always be careful when downloading user-created maps off of Battle.net, because you could get a virus, even on a Mac.
As for your iMac crashing 2 times, Macs crash, almost as much as PCs and it is going to happen on that Macbook Pro regardless of whether Warcraft 3 is installed or not, so it isn't a scapegoat.
In the end, I recommend that you do not argue with your parents, but discuss it. If it has gotten to the point where it is a full-blown argument, then it is not worth it as your parents ultimately have a right to bar anything from their computer that they purchased regardless of whether it makes sense or not.
My advice? Ask them to buy you a decent gaming rig and say goodbye to Warcraft 3 and get into some REAL gaming. A very good gaming rig starts at about $800, such as the iBuypower Gamer Fire 480.