To recap:
-Classic won't work since you're running an Intel. Diablo under Classic (on PPC) works poorly or not at all anyway, in my experience.
-You can try SheepShaver. However: it can be confusing to set up (the linked directions @Xeem are pretty good); you need an OS 8 or 9 system folder/disc to run it (NOT downloadable from Apple); and it's pretty unstable for gaming in any case - this on my PPC, which runs it natively.
-The Windows solution. Since the Battle Chest CDs are hybrids, you'll have a PC version to run. However, you'll have to buy a copy of Parallels/VMWare. Why not just do Boot Camp for free? Even then, you'd still need (have to buy?) a copy of Windows XP. A little pricey to run one old game.
Short answer: there's no one solution that's both easy and free, sadly.
I can understand why you'd want to play Diablo, though. #II has more choices, but seems to devolve into a constant flood of A. weak items for your class or B. great items for another class, requiring you to town-portal every 5 minutes to free up space. Diablo really had the formula down of streamlined dungeon crawling, nearly every item an improvement, and click-click-click gaming.