well... for one, ET isn't really made with Mac in mind in the first place I believe.
Also, I don't believe ET will kill your Mac. The fact that it doesn't know what to do with the files, may cause some things to slow down on your mac, or the mac to crash, but it shouldn't give any permanent damage to your system.
Trying to get it to work directly on your Mac... I can't really give tips on that.
Perhaps, run a virtual windows computer on your system (Mac should have the power to keep that going while running the game) Install ET there, and that should just run like normal.
Seeing as the windows virtual pc will know what to do with the files, and even if that should crash, it should only crash the virtual pc, not the actual mac.
That's how I see it at least =]